Skip to main content
U.S. flag

An official website of the United States government

Official websites use .gov
A .gov website belongs to an official government organization in the United States.

Secure .gov websites use HTTPS
A lock ( ) or https:// means you’ve safely connected to the .gov website. Share sensitive information only on official, secure websites.

World Trade Center Investigation


The collapse of the WTC buildings following the terrorist attacks on September 11, 2001, was one of the worst building disasters in US history, killing 2,749 people, including 400 emergency responders.

NIST responded to calls from Congress and the public to carry out a federal investigation of why the buildings collapsed, the evacuation of building occupants and the emergency response. NIST was given this authority when Congress passed, and the President signed into law, the National Construction Safety Team Act of 2002. 

More than 200 professionals and technical subject matter experts, including 85 NIST staff members, answered the call and participated in the investigation.

As part of the investigation, the NIST team gathered every bit of evidence they could find. Activities included:

  • Interviews with 1,056 surviving occupants of the WTC buildings and 116 emergency responders.
  • Extensive reviews of design, construction, maintenance and inspection documents for the buildings 
  • Examination of hundreds of structural steel components from WTC buildings
  • Gathering and analyzing thousands of pieces of video, photographic, and audio evidence from professional sources and the public
NIST's World Trade Center Investigations
NIST's World Trade Center Investigations
Learn more about NIST's work on the World Trade Center investigations, from how it conducted the study to the recommendations and their lasting legacy.

The Reports

Throughout the process, NIST held 23 public meetings and provided multiple opportunities for the public to review and comment on drafts of the reports. In addition, all of the reports were reviewed by an external, non-NIST NCST advisory committee.

In the end, NIST released final versions of the 43 reports on the WTC towers, totaling some 10,000 pages, on October 26, 2005. NIST released final versions of the three reports on WTC 7, totaling about 1,000 pages, on November 25, 2008.

NIST is aware of other studies related to the WTC collapse and stands by its original findings. 

The Legacy

Building and Fire Codes, Standards and Practices

The WTC investigation has had a significant legacy. In the reports, NIST made 31 recommendations for improvements to building and fire codes, standards, and practices based on the WTC investigation. While the federal government has no regulatory authority for building and fire codes, many U.S. codes and standards were subsequently updated to improve areas such as structural integrity, fire resistance, occupant evacuation and emergency responder communications. NIST’s recommendations have had a significant impact on design and construction practices for high-rise buildings worldwide, including the new buildings at the rebuilt World Trade Center site. 

In addition, NIST scientists conducted tests of steel from the WTC buildings to measure their mechanical properties at normal and elevated temperatures. These tests led to the development and validation of performance criteria for fire resistive steel. This type of steel, which was not available at the time the WTC was built, would not necessarily prevent a building from collapsing during a fire, but it would give occupants more time to escape. 

Read Jason Averill's story about investigating the WTC evacuation and Steve Kirkpatrick's work analyzing the aftermath of the Twin Towers aircraft impacts.

DNA Identification

NIST also played a role in the effort to identify the victims of the 9/11 attack. This effort was at the time — and still is — the largest forensic identification effort ever undertaken. This effort was particularly challenging because in many cases the DNA of the victims was severely degraded by exposure to intense heat from burning jet fuel, as well as moisture and decay in the weeks and months following the attack. To make an identification, experts analyzed segments of DNA called forensic markers. When DNA degrades, these forensic markers break apart, which makes them difficult to analyze.

To cope with this challenge, researchers at NIST, Ohio University and Bode Technology developed new forensic markers involving shorter segments of DNA, which by virtue of their small size are more likely to remain intact as the DNA degrades. This effort allowed forensic experts to identify thousands of remains that otherwise would have never been returned to their families. The research conducted at NIST in the aftermath of 9/11 led to the development of new “mini-markers” that are now routinely used in criminal casework. These markers have helped investigators solve countless sexual assaults, homicides and other crimes that would otherwise have gone cold.

Read John Butler's reflections on assisting with the 9/11 World Trade Center DNA identifications.

Public Safety Communications

Among the victims were more than 400 first responders, including firefighters, police officers, and Port Authority officers. These deaths revealed a huge gap in public safety communications and fundamentally changed NIST’s research focus in public safety communications. During the disaster, first responders struggled to communicate due to lack of interoperability among the radio systems and over-crowded communication channels. As a result, the 9/11 Commission made communications research for public safety a priority, and tasked NIST with finding solutions.

NIST established its Public Safety Communications Research division and became a technical advisor to Congress and the White House on issues surrounding public safety communications. This new research focus helped established FirstNet, a nationwide broadband network dedicated to first responder communications. In the 20 years since the attacks, NIST and its partnering research organizations and industry have resolved many of the radio interoperability issues and now focus on modernizing communications technology for first responders.

Read Dereck Orr's reflections on how 9-11 changed him, and public safety communications.

National Fire Research Laboratory & Disaster Resilience Program

As a result of the WTC investigation, NIST established the National Fire Research Laboratory, which has provided a unique capability to test performance of large-scale structures under realistic conditions. NIST also established a Disaster Resilience Program, focused on ways to improve the safety and resilience of buildings and communities in the face of multiple threats including wildfires, hurricanes, tornadoes and earthquakes. Improving resilience has emerged as a major theme of the 21st century, and the NCST Act, created in response to 9/11, has enabled NIST to conduct technical investigations of subsequent significant disasters. including the Joplin, Missouri, tornado in May 2011; the 2003 Station nightclub fire in West Warwick, Rhode Island; the effects of 2017’s Hurricane Maria on Puerto Rico and, most recently, the Champlain Towers Collapse in Miami, Florida, in June 2021.