The aim of this article is to examine aspects of the attacks and the emergency response through a human factors lens. Given the wealth of information available, I can only provide a brief summary here. As with the other incident summaries on this website, I’m conscious that years of detailed analysis and reports since the attacks provide us with a unique insight that would not have been available prior to September 11.

“As we turn to the events of September 11, we are mindful of the unfair perspective afforded by hindsight”

The 9/11 Commission Report, p.285

An overview of the attacks

On September 11, 2001, terrorists hijacked four commercial airplanes from three different airports, as part of a co-ordinated attack against the United States. Two of these planes crashed into the World Trade Center Twin Towers in New York City and one hit the Pentagon near Washington, D.C. The fourth plane crashed in a field in southwestern Pennsylvania, the target presumably the White House or the Capitol. More than 3,000 people were killed in these terrorist attacks. The number of first responders who have since died from World Trade Center-related illnesses has surpassed those who died on the day.

Response to hijacking

Previous hijacking of commercial aircraft followed a general pattern and there was a defined response for two federal agencies – the Federal Aviation Administration (FAA) and North American Aerospace Defense Command (NORAD). Air traffic controllers track aircraft using their unique transponders. But on three of the four hijacked aircraft the transponders were turned off, making them more difficult to track (visible only when crossing the path of a ground-based radar). Hijacked aircraft did not usually attempt to disappear.

The game plan for hijackings usually allowed time for the FAA and NORAD to respond according to established protocols, but in this case the military had less than 9 minutes’ warning before the first airliner hit the World Trade Center and 25 minutes before the second.

“On the morning of 9/11, the existing protocol [for responding to hijacking] was unsuited in every respect for what was about to happen”

The 9/11 Commission Report, p.18

The air traffic controllers on September 11 had to improvise their response to hijacked aircraft that attempted to disappear and the military were not fully prepared for suicide hijackers who intended to use commercial aircraft as weapons. Confusion and failed communications between the air traffic controllers and the military hampered the response, including the coordination of fighters sent to intercept the hijacked aircraft.

The FAA did not have “specific and credible” intelligence of a plot directed at civil aviation. NORAD, which had responsibility for defending U.S. airspace, was focussed on threats coming from overseas (the dominant threat being cruise missiles). On September 11, together with the FAA, they were unprepared for the type of attacks they now faced. The circumstances were very difficult, and had not been encountered before. These were situations for which the agencies had never trained to meet.

“We do not believe that the true picture of that morning reflects discredit on the operational personnel at NEADS or FAA facilities. NEADS commanders and officers actively sought out information, and made the best judgments they could on the basis of what they knew. Individual FAA controllers, facility managers, and Command Center managers thought outside the box in recommending a nationwide alert, in ground-stopping local traffic, and, ultimately, in deciding to land all aircraft and executing that unprecedented order flawlessly”

The 9/11 Commission Report, p.31

The response on board the aircraft

Gaining unauthorised entry to the cockpit did not appear to be a challenge for the hijackers. The cockpit doors were not strong enough to prevent a forced entry. At the time of the hijacking, all American Airlines flight attendants carried cockpit keys. On United flights, the keys were stored in the cabin. Cockpit security was also compromised by the use of common locks (one key operated the cockpits of all Boeing aircraft).

Flight crews on the four aircraft responded bravely. But they were unprepared for the situation they found themselves in on September 11. The assumption would be that hijackers of commercial aircraft would have demands, such as for asylum or the release of prisoners. Based on previous events, the FAA taught flight crews that the aircraft would land at an airport and then negotiations would occur.

There was no training or preparation for responding to violence from hijackers, or response to a suicide hijacking. Aircrews were trained to refrain from negotiating with hijackers, or trying to overpower them.

Other controls in place included the use of armed federal air marshals. However, a total of only 33 marshals were available, and they were not deployed on any U.S. domestic flights, such as those involved in this attack.

Emergency response on the ground

Learning from previous events

A previous attack on the World Trade Center exposed vulnerabilities in emergency preparedness. On 26 February 1993, a bomb was detonated in the parking garage beneath the Twin Towers. Six people were killed and thousands were injured. The crater was 100-foot wide and several stories deep.

This incident revealed a number of failures that affected the emergency response:

• The towers lost power and communications capability
• Generators had to be shut down to ensure safety
• The public-address system and emergency lighting systems failed
• The stairwells were unlit, and when combined with smoke, made them very difficult to navigate
• Fire Department of New York (FDNY) radios did not function in buildings as large as the Twin Towers
• The 911 emergency call system was overwhelmed.

The 9/11 Commission Report notes that significant upgrades were made to address these problems, and these changes greatly assisted in the evacuation of the Twin Towers on September 11.

Fire drills in the World Trade Center

These drills were conducted at least twice per year. Fire wardens would lead everyone into the centre of the floor where they would wait for further instructions. However, during these drills workers were not directed into the stairwells, or informed of their configuration. Full or partial evacuation drills were not held. It is noted that in the response to the 1993 bombing, many of the injuries occurred during the evacuation.

The 9/11 Commission states that all individuals should make themselves familiar with every stairwell in their workplace and be prepared should disaster strike (p.318).

First Responders: Control, coordination and communication

There were four organisations involved in responding to the World Trade Center on 9/11:

1. Fire Department of New York (FDNY)
2. New York Police Department (NYPD)
3. Port Authority Police Department (PAPD)
4. Mayor’s Office of Emergency Management (OEM).

Numerous NYPD officers saw an aircraft hit the North Tower at 08:46am and informed their control centre. The NYPD Aviation Unit was dispatched at 08:50am and en-route they informed air traffic controllers of a commercial airplane crashing into the World Trade Centre (who at this point were unaware of the event). The Aviation Unit quickly determined that rooftop rescues would not be possible due to flames, heat and smoke. NYPD cleared transit routes around the city to support the movement of emergency vehicles. NYPD and the PAPD coordinated the closing of bridges and tunnels into Manhattan.

The OEM activated the Emergency Operations Center within a couple of minutes of the North Tower being hit. The different agencies attempted to work together, but the FDNY and NYPD had separate command posts and no reliable means of communicating between them.

There was minimal coordination between the FDNY and NYPD throughout the response, and limited communications between the senior officers of these agencies.

More than 200 fire units and 100 ambulances responded. As the attacks were around the time of normal shift changes, many off-duty personnel responded with their units. The number of first responders (both on-duty, off-duty and volunteers) meant that Chiefs did not have an accurate view of the number and location of resources available. The recall of off-duty personnel was disorganised, having not been activated for over 30 years and there had been no training in its activation. Most senior civilian FDNY personnel also attended the scene, but many had no role or responsibility in the response.

Many fire units did not report to staging areas on arrival, preventing them from gaining information and orientation before entering the towers. This also meant that Chiefs were unaware of their location and deployed additional units that may not have been required at that time.

The height of the two buildings (110 floors above concourse level) severely hampered any emergency response. Members of the FDNY climbed the stairwells, laden with protective equipment and breathing apparatus, and carrying other heavy equipment. Wearing full gear, firefighters climbed the stairs at a rate of approximately two minutes per floor. This was extremely hard work, and yet some firefighters climbed to floors in the 40s. However, their ability to conduct rescue and firefighting operations was greatly diminished as they climbed.

The egress of building occupants in the relatively narrow stairways created difficulties for emergency responders:

• The lower-level doors only permitted one person to enter or exit at a time, and so an occupant leaving the stairway prevented responders from entering the stairway.
• The flow of occupants on the stairs caused response teams to become separated.
• Building occupants coming down the stairs made it difficult for responders to move equipment up the stairs.

As FDNY units climbed the towers, their ability to communicate with their chiefs was poor, because of the limits of their radios in high-rise buildings, and because so many firefighters were attempting to use radios at any one time. Coordination of FDNY personnel was further hampered because several radio frequencies were in use. The FDNY and NYPD used different radio frequencies and so couldn’t share information easily, or gain the same situation awareness. Also, some first responders were off-duty, and didn’t have access to radios at all.

“Significant shortcomings within the FDNY’s command and control capabilities were painfully exposed on September 11”

The 9/11 Commission Report, p.320

Notably, NYPD had fewer command, control and communication issues, partly because they had experience of mobilising thousands of officers for crowd control at major events. However, in all of the crash sites on 9/11, there were difficulties in radio communications between the various first responder agencies. As a result, coordination between agencies was often inadequate.

The crash of Flight 175 into the South Tower greatly escalated the command, control and communication difficulties. However, despite the unimaginable challenges, individual first responders assisted thousands of workers to evacuate the towers – and in doing so enabled numerous people to survive. There were many heroic acts that day.

Coordination between the various first responder agencies was hampered by a lack of information sharing between these agencies. For example, they did not coordinate floor searches, and therefore redundant searches were conducted. They did not share an incident command post. It wasn’t clear to first responders which federal agency was in charge at the World Trade Center, or the nature of the relationship between federal and local response authorities. This lack of clarity led to confusion and coordination difficulties among those participating in the response.

“There was a lack of comprehensive coordination between FDNY, NYPD, and PAPD personnel climbing above the ground floors in the Twin Towers”

The 9/11 Commission Report, p.321

There are reports of cultural tensions between agencies; for example, when NYPD officers in the North Tower advised evacuation, some first responders refused to take instructions from Police Officers.

The Pentagon response

The emergency response to the attack on the Pentagon (Flight 77) was considered a success for several reasons:

• The first responder agencies had extensive prior experience of working together – they had good professional working relationships.
• An Incident Command System was in place (a formalised management structure for emergency response).
• The incident command post had a clear view of the crash site.
• The Incident Commander ordered an evacuation of the impact area (as a partial collapse was imminent).

No first responder was injured at this site. Obviously, the Pentagon incident site was less complex, and a much easier site to secure than the World Trade Center. However, the response at the Pentagon encountered similar issues to that at the WTC: for example, radio channels were overwhelmed, and incident command was complicated as organisations and individuals self-responded without the coordination of the Commander.

The importance of information

Command and control decisions were significantly affected by the lack of information. FDNY Chiefs reported afterwards that they did not feel they had sufficient information to assist in their decisions and response. In particular, they did not receive reports from the NYPD helicopters. They had little information on the extent of damage to upper floors and they did not know whether the stairwells were intact or not.

After the South Tower collapsed, first responders in the North Tower were likely unaware of what had happened to the neighbouring building. All FDNY command posts had been abandoned during that collapse. Many FDNY Chiefs did not learn of the South Tower collapse until 30 minutes after the event. Although the NYPD Aviation Unit witnessed the collapse of the South Tower and advised evacuation of the WTC complex, communications were hampered by overwhelming transmissions on radio channels.

“People watching on TV certainly had more knowledge of what was happening a hundred floors above us than we did in the lobby. . . . Without critical information coming in . . . it’s very difficult to make informed, critical decisions”

FDNY Chief, The 9/11 Commission Report, p.298

Some FDNY Chiefs gave evacuation orders to all units in the North Tower (at this point not all of them knowing that the South Tower had collapsed). Most of the evacuation instructions did not mention that the South Tower had fallen. Several firefighters who survived the North Tower reported that they would have evacuated more urgently if they had known of the collapse of the South Tower.

Situation awareness is a key human factors topic. I define it on the relevant topic page as “Developing and maintaining a dynamic awareness of the situation and the risks present in an activity, based on gathering information from multiple sources from the task environment, understanding what the information means and using it to think ahead about what may happen next”. When those coordinating the response had poor situation awareness, this impacted the quality of their decisions and actions.

Following the impact to the North Tower (WTC 1), the building communication system used for emergency announcements to the occupants was inoperable, and the Warden phones were not working. This prevented any building communications to the upper floors.

The large number of first responders (including many not officially on duty) made it difficult to track the location and welfare of emergency responders, and the number of casualties created difficulties in tracking patients.

The 911 system and 911 operators

Several emergency intercoms were disabled by the impact of the aircraft, or they could not be reached by building occupants, and so many people called “911” (the emergency number in the United States). This system was not equipped to handle the enormous volume of calls that were received. Standard procedure was that any calls relating to fires would be transferred to the FDNY dispatch, but this process was also impacted by the number of calls.

The 911 operators and FDNY dispatchers had no information on either the location or extent of the fires, and were therefore unable to tell callers whether they were above or below the fire. These operators and dispatchers were also unaware of information from the NYPD Aviation team that rooftop rescues by helicopter would be impossible. If they knew this, they could have passed this information onto callers who considered climbing up towards the roof.

“This lack of information, [about the inability to conduct rooftop rescues] combined with the general advice to remain where they were, may have caused civilians above the impact not to attempt to descend, although stairwell A may have been passable”

The 9/11 Commission Report, p.295 (following the impact to the South Tower)

At least one person descended from the 91st floor of the South Tower (Flight 175 crashed through the 77th to the 85th floors) and stairwell A was reported to be almost empty. In the North Tower, where Flight 11 impacted floors 93 to 99, at least one stairwell was clear from the upper 80s down. If they were able, and decided to evacuate, many occupants of both towers had a clear route and sufficient time to leave the building.

It is recommended that the integration of different first responder agencies into a coordinated incident response should include 911 operators.

Confusion over evacuation

Although the evacuation of thousands of people from skyscrapers is highly problematic, the FDNY chiefs arriving on scene quickly determined that all building occupants in the North Tower should evacuate immediately. They gave the same advice for South Tower occupants at 8:57am (judging that the impact on the North Tower made the whole WTC complex unsafe, not because of concerns of further attacks).

The initial response by FDNY after the first impact (to the North Tower) was that this would be a rescue operation:

“We were going to vacate the building, get everybody out, and then we were going to get out”

FDNY Chief, The 9/11 Commission Report, p.291

Unfortunately, these decisions to evacuate were not communicated to the 911 operators or the FDNY dispatchers. For the next hour, regardless of their location, workers in both towers were generally instructed to stay low, remain where they were and wait for emergency services to reach them. Despite clear stairwells for many occupants, and plenty of time to leave, people remained on their floor, waiting for help to arrive. But the traditional ‘rules’ did not apply in this unprecedented and complex situation.

Despite advice from the 911 or FDNY operators to stay put, most workers in the North Tower began to evacuate.

Many people in the South Tower also decided to leave the building. A major tenant of the South Tower, Morgan Stanley, occupying more than 20 floors, evacuated its employees (a decision that saved many lives). However, an announcement over the South Tower public address system at 08:49am advised that an incident had occurred in the North Tower and advised people to remain in their office, or return to their floor. As a result of this announcement, many people remained on their floor, or if evacuating, went back to their offices.

At 09:00am, the PAPD commanding officer of the WTC ordered an evacuation of all civilians across the WTC complex. This instruction was broadcast on a channel that could not be heard by the South Tower deputy fire safety director.

A subsequent announcement in the South Tower at 09:02am (less than a minute before the building was hit) advised workers to commence evacuation if conditions warranted it. These announcements were not fully consistent with written protocol or emergency instructions.

Due to the heroic efforts of first responders, the vast majority of workers below the impact zones were able to evacuate the towers in under one hour (compared to more than four hours following the 1993 bombing). Approximately 15,000 people successfully evacuated the two towers.

The importance of design

My experience of auditing emergency response capabilities on oil and gas installations, together with my role in the design of escape and evacuation routes on offshore facilities, led me to explore how design of the WTC buildings may have influenced the outcomes.

It’s interesting to note that very simple measures installed following the 1993 bombing greatly increased evacuation and survival; for example, the use of glow strips attached to handrails and on the stairs. Many building occupants reported that these basic improvements were key to their survival.

However, the design of the stairwells impaired evacuation. For example, smoke doors on landings led some to believe that the stairway had ended. Other people were confused by stairwell deviations – evacuation required crossing several landings from one stairwell to another (often more than 30m apart). These physical design aspects were not expected by all of those leaving the building. As mentioned above, people had not been shown the stairwells as part of their emergency drills. Occupants found these horizontal transfers (with multiple 90 degree turns) confusing in the 1993 evacuation.

During the design of the Twin Towers, changes in the NYC building codes reduced the number of emergency exit stairwells in each building from six to three. And the design placed all three stairwells close to one another, in the central core of the building. This reduced evacuation success in an event that impacted on the central core. The new design rules also relaxed the previous requirement for a ‘fire tower’ – a stairwell encased in masonry, and so the WTC did not have these.

On September 11, with all three stairwells in the North Tower destroyed, office workers on the floors above the impact zone were beyond help. The WTC did not have a plan for how people would evacuate if the stairwells were not passable.

In desperation, hundreds of people fell or jumped from over a thousand feet.

Failures prior to 9/11: Intelligence failures

The Congressional Joint Inquiry, the Office of the Inspector General (OIG) review of the Federal Bureau of Investigation (FBI), and the independent 9/11 Commission all discussed failures by the intelligence community. These failures are stated to be the result of substantial systemic and long-standing deficiencies.

There were several missed opportunities to prevent the 9/11 plot. The Commission discusses information not being shared, analysis not being pooled and a divide between the foreign and domestic agencies. It also highlights failures during the airport security screening process, such as a focus on identifying potential bombers, rather than potential hijackers. (There is a Recommendation to conduct a human factors study aimed at improving screener performance, see p.393 of the Commission Report).

“When these attacks occur, as they likely will, we will wonder what more we could have done to stop them”

Message from Richard Clarke, National Security Council (NSC) counterterrorism coordinator, to National Security Advisor Condoleezza Rice, 29 May 2001 (3 months before the 9/11 attacks)

Chapter 8 of the 9/11 Commission Report is titled “The system was blinking red“, underlining a statement made by the Director of Central Intelligence to the Commission. Although there were frequent reports of potential terrorist activities discussed by counterterrorism officials, the level of reporting on threats and planned attacks increased dramatically in the spring of 2001. The number and severity of threat reports at this time is described by the intelligence community as unprecedented.

However, when threats were assessed, it was generally assumed that attacks would be overseas. As a result, certain U.S. embassies overseas were closed, and military assets were relocated away from potential harm.

“The September 11 attacks fell into the void between the foreign and domestic threats. The foreign intelligence agencies were watching overseas, alert to foreign threats to U.S. interests there. The domestic agencies were waiting for evidence of a domestic threat from sleeper cells within the United States. No one was looking for a foreign threat to domestic targets”

The 9/11 Commission Report, p.263

There were several leads under investigation in the months prior to the 9/11 attacks, but these were not necessarily linked to each other, or to the increasing threat reports. This explains the media reports after 9/11 that describe the intelligence community as failing to “connect the dots”. But these failures can be seen as symptoms, rather than the disease.

To continue the medical analogy, the 9/11 Commission Report describes the various intelligence agencies as acting like the specialists in a hospital – they each order tests, look for symptoms and prescribe treatment. However, what was necessary is someone who makes sure that these specialists all work together as a coordinated team.

The Commission discusses a “failure of imagination” – partly for the fact that the use of aircraft as weapons had not been robustly considered by the intelligence community. Attention was focussed on hijackings, or the detonation of explosives onboard aircraft. (Previous hijackings were undertaken in order to stimulate dialogue or negotiation, such as the release of hostages). Co-ordinated multiple hi-jackings had never happened before.

The use of aircraft as weapons (a suicide hijacking) had been discussed in the intelligence community, but this would have been one possible theory amongst thousands of terrorist threats under consideration.

“In one conversation between a Minneapolis supervisor and a headquarters agent, the latter complained that Minneapolis’s FISA [search warrant] request was couched in a manner intended to get people “spun up”. The supervisor replied that was precisely his intent. He said he was “trying to keep someone from taking a plane and crashing into the World Trade Center”.

The 9/11 Commission Report, p.275

There’s a chapter in the 9/11 Commission Report called “Foresight – and Hindsight”. In the introduction, the Report notes that hindsight is both a benefit and a handicap. With everything that we know today, it is easy to see what the various warning signals may have been pointing to. However, before a disaster, it is much harder to understand which signals are relevant, or what they are signalling. This potential for hindsight bias is recognised by the Commission.

It’s easy with hindsight to say this or that should have been done, or that the various warnings were clear. But the different agencies had their own roles and directives, operated within the resources that they had available and intelligence officers were drowning in information.

“With that caution in mind, we asked ourselves, before we judged others, whether the insights that seem apparent now would really have been meaningful at the time, given the limits of what people then could reasonably have known or done”

The 9/11 Commission Report, p.339

I note that the US “intelligence community” is composed of more than a dozen different agencies. The intelligence community most likely suffered from the same issues that are common to most large companies. Organisational complexity has been implicated in previous events, for example the NASA organisation and the UK Ministry of Defence.

Final reflections on 9/11

There were many human factors topics relevant to 9/11, including communications, procedures, training, situation awareness, design and leadership. Similar to several other events explored on these pages (see particularly Space Shuttles Challenger and Columbia and Nimrod XV230), the 9/11 Commission places emphasis on institutional failings.

The tragedies of 9/11 have shown that previously unimaginable events can occur, and whilst we may not be able to predict the exact nature of future events, all individuals – as well as first responders – need to be prepared.

“A rededication to preparedness is perhaps the best way to honor the memories of those we lost that day”

The 9/11 Commission Report, p.323

Related articles

Further reading

The 9/11 Commission Report: Final Report of the National Commission on Terrorist Attacks Upon the United States, U.S. Government Printing Office, July 2004. The official Report provides a full and complete account of the circumstances surrounding the September 11th, 2001, terrorist attacks, including preparedness for and the immediate response to the attacks. It also includes recommendations designed to guard against future attacks. (This report is freely available as a pdf, however I purchased a hard copy for easier reading).

www.archives.gov/research/9-11. This is the official site for the 9/11 Commission records, hosted by the U.S. National Archives and Records Administration (NARA).

Mitchell Zuckoff, Fall and Rise: The Story of 9/11. I purchased this book to complement the official version of events in the 9/11 Commission Report. It tells the stories of many people affected by the events of 9/11, from those in the hijacked aircraft to those on the ground (both occupants of buildings and emergency responders).

Final report on the collapse of the World Trade Center towers, National Institute of Standards and Technology (NIST), 2005. This report describes how the aircraft impacts and subsequent fires led to the collapse of the towers; including an evaluation of the building evacuation and emergency response procedures; what procedures and practices were used in the design, construction, operation, and maintenance of the towers; and areas in current building and fire codes, standards, and practices that warrant revision.

The Emergency Response Operations, National Institute of Standards and Technology (NIST), 2005. This report addresses the operations of the emergency responders, the technologies used during WTC operations, and the guidelines and practices that governed these operations.

Occupant Behavior, Egress, and Emergency Communications, National Institute of Standards and Technology (NIST), 2005. This report describes the occupant evacuation of World Trade Center (WTC) 1 and WTC 2 on September 11, 2001. It describes the egress system (stairwells and elevators), the emergency procedures in place and outlines the process of evacuation from these two buildings.

Header image credit: U.S. Customs and Border Protection (Andrea Booher Federal Emergency Management Agency, FEMA), https://www.cbp.gov/medialibrary/collections/37672