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Taking Measure

Just a Standard Blog

20 Years Later: NIST's World Trade Center Investigation and Its Legacy

collage showing rubble from the collapse of the towers, a mock up of a section of the towers, scientists looking at structural steel and the words Building and Fire Codes
Credit: N. Hanacek/NIST

The collapse of the World Trade Center (WTC) buildings following the terrorist attacks of Sept. 11, 2001, was one of the worst-ever building disasters in recorded history — killing 2,749 people. More than 400 emergency responders were among those killed, the largest loss of life for this group in a single incident.

I was shocked to learn of the 9/11 terrorist attacks, along with the rest of the nation, while attending a conference in Krakow, Poland. My immediate thoughts that Tuesday turned to the safety of my family back home. Once I talked to them and knew they were safe, I decided to fly to Berlin since that would give me more options to get home quickly once flights resumed over U.S. airspace. Over breakfast at the Berlin hotel that Friday, I began sketching out initial ideas for our response that would involve a reconnaissance and assessment phase followed by a more detailed investigation.

A few weeks later, other National Institute of Standards and Technology (NIST) leaders and I began briefing policy leaders at the Department of Commerce, the parent agency of NIST. We explained the ways in which our agency could help determine the technical reasons why the buildings collapsed — and how this knowledge could not only provide answers for all those affected by the WTC disaster but also help make all buildings safer in the future. Meeting with House and Senate congressional staff, we discussed the lack of adequate authorities for any government agency, not just NIST, to conduct technical investigations into such building collapses. We reminded them about NIST’s long history and expertise in conducting disaster and failure studies, from the Gulf Coast structures destroyed by Hurricane Camille in 1969 and the 1971 San Fernando earthquake to the collapse of the Kansas City Hyatt Regency Hotel walkway in 1981 and the collapse of Connecticut’s L’Ambiance Plaza building in 1987.

After numerous briefings and two hearings at which the NIST director and others testified, Congress enacted, and the president signed into law, the National Construction Safety Team (NCST) Act of 2002. This law, for the first time, gave NIST a comprehensive set of authorities to conduct technical investigations in the wake of any building failure that resulted in substantial loss of life or posed a significant threat of doing so. Congress directed $16 million to support the NIST investigation as part of an emergency supplemental appropriations for the 2002 fiscal year.

I had the unique opportunity and privilege to be engaged in every aspect of the investigation from its conception, planning and budgeting to its execution, communication and the implementation of its recommendations via standards, codes and industry organizations. I was appointed to lead the WTC investigation while serving as chief of the Structures Division, just seven years after I had joined NIST from the Massachusetts Institute of Technology (MIT). I embraced this immense new responsibility to help find out what happened on that day and what, if any, actions should be taken to make our buildings more resilient.

The NIST team for the WTC investigation included approximately 85 staff members from across the agency. But the investigation was not done only by NIST. The full investigation team consisted of more than 200 people, including world-class scientists and engineers from industry and academia, to help us meet the challenges of the investigation’s unprecedented scope, scale and technical complexity. The investigation team acted tirelessly with dedication to and in memory of the lives that were lost on that day, and with the complete objectivity and integrity that have guided NIST over its 120-year history. Our investigation also benefited from the new NCST Advisory Committee established by Congress and made up of highly respected non-NIST experts from industry and academia.

The WTC buildings collapsed from many different factors working together in a complex fashion. Our investigation used physics-based computer simulations together with the available evidence. These very large-scale computations took into account the multiple impacts of airplane and debris, the spread of multi-floor fires ignited by jet fuel or flying debris, the heat-related weakening of structural components with intact or dislodged fireproofing materials, and the complex responses of the building structures leading to their collapse. Successfully combining state-of-the-art computational tools in ways that had not been done before, a single end-to-end simulation on then-existing computing resources took about two months to carry out for one of the WTC towers and eight months for WTC 7. We conducted live fire experiments that ranged from the simple (to gather data on burning characteristics of the jet fuel) to the more complex (a complete WTC office mock-up with furnishings). These experiments validated the computer simulations that would ultimately determine the probable collapse sequence for each building and guide many recommendations.

We relied on extensive evidence — combined with careful and detailed review, analysis and testing — to build our models, and validate our simulations, hypotheses and findings. This evidence included 236 major structural steel components, 7,000 video segments, more than 7,000 photographs, emergency responder communications, published accounts of evacuation from 400 survivors, and a large collection of design, construction, maintenance and inspection documents for the three buildings.

We conducted extensive interviews with more than 1,000 surviving occupants of the World Trade Center buildings to analyze occupant behavior and evacuation during emergencies. We also interviewed 116 emergency responders to document and assess their operating protocols and wireless communications in large-scale events and challenging radio-frequency environments such as those encountered in the WTC buildings.

We coordinated with the National Commission on Terrorist Activities Upon the United States (the 9/11 Commission) and the City of New York in the planning and conduct of emergency responder interviews. This enabled 9/11 Commission staff to participate in these interviews alongside NIST staff. We also engaged extensively with local authorities and key stakeholders in New York City, including the families of victims and professional organizations, and with other federal agencies such as the Federal Emergency Management Agency (FEMA) and the National Transportation Safety Board (NTSB).

The NIST team ensured that the investigation was conducted in an extremely open and transparent manner given the intense interest of the public — especially families of victims, local authorities and industry — who held many different perspectives on the reasons for the building collapses and loss of lives. We met regularly with diverse stakeholder groups, soliciting and considering their input in developing the investigation plan, findings, recommendations and final reports. We provided detailed updates and answered wide-ranging questions at 23 public meetings and briefings (with seven in New York City), issued seven news releases and media updates, and maintained a comprehensive, publicly accessible website during the investigation.

As a result of the exceptional teamwork in our investigation, we were able to overcome the many technical and nontechnical challenges, while maintaining the highest degree of quality, objectivity and credibility. We are deeply grateful to all of the people who worked with NIST, provided photos and other critical information, and gave unstintingly of their time to support and help us successfully carry out the largest U.S. building failure investigation ever conducted.

NIST released 47 reports from our investigation — totaling about 11,000 pages — which included robust science-based findings and conclusions, as well as recommendations for major safety improvements to U.S. buildings. We determined why and how each of the 110-story WTC towers collapsed following the initial impacts of the aircraft and why and how the 47-story WTC 7 collapsed; whether the injuries and fatalities were high or low depending on location, including all technical aspects of fire protection, occupant behavior, evacuation and emergency response; and what procedures and practices were used in the design, construction, operation and maintenance of these buildings.

The NIST recommendations resulted in more than 40 major and far-reaching changes to U.S. building and fire safety codes to improve the safety of buildings, their occupants and emergency responders. These recommendations were not designed, however, to make buildings withstand aircraft impact. It would be better instead to keep terrorists away from airplanes, and airplanes away from buildings.

Our recommendations have already had significant impact on design and construction practice for high-rise buildings in New York City, across the U.S., and around the world. We now see evidence of more resilient construction such as wider stairways with hardened enclosures, increased fire resistance rating for structural frames, significantly improved bond strength for fireproofing materials so that it is difficult to dislodge them from the structural steel they are protecting, and more robust radio communications coverage within buildings for emergency responders. 

Now, 20 years later, I have found that this event has had a profound effect on the arc of my career and life’s work by opening up many new opportunities to advance impactful research and innovation for our nation. The impact our recommendations have had on U.S. building and fire codes and standards has been our greatest accomplishment, especially seeing as the federal government does not have any regulatory authority over them. Instead, standards and codes development organizations quickly adopted them. Changes to the nation’s building and fire codes and standards — stemming from our proactive efforts to engage the standards and codes bodies — were history-making due to the extraordinary magnitude of the safety improvements, uncharacteristically rapid speed with which such major changes were adopted (less than five years), and exceptional support of the nation’s building and fire safety officials. These changes have significantly advanced the safety and protection of America’s buildings, their occupants and emergency responders in future disasters.

NIST rarely — if ever — had experienced such active public and media interest during — and after — an investigation or study. We have reviewed other WTC studies that have come out since we completed our investigation, including those that consider alternative hypotheses for the WTC building collapses. Based on our exhaustive analysis and the evidence available to us, NIST continues to stand behind the findings and recommendations of our investigation.

The WTC investigation also had a profound impact on NIST itself. Based on our experience from the WTC investigation and our new investigative authorities, we began a formal NIST program in 2010 to coordinate disaster and failure studies. This program has allowed NIST to respond more rapidly and effectively to incidents such as the May 2011 tornado in Joplin, Missouri, and the June 2021 collapse of the Champlain Towers South Condominium in Surfside, Florida.

It also enabled NIST to initiate, develop and establish a world-class disaster resilience program beginning in 2007. This program is transforming the safety of buildings and infrastructure systems to a comprehensive community-based approach. In recognition of our WTC work, Congress designated NIST as the lead federal agency to oversee the $130 million per year, multi-agency U.S. National Earthquake Hazards Reduction Program (NEHRP) in 2004. Congress further enacted the National Windstorm Impact Reduction Act in 2004 and designated NIST as its lead agency in 2015. We have achieved significant advancements in structural fire engineering practices through new standards and guidelines. This effort continues via a unique new NIST facility — the National Fire Research Laboratory — to test the performance of real-scale structures under realistic fire and structural loading. Today, our disaster resilience program is addressing the risks of climate change to our communities due to the increasing frequency and severity of hurricanes, tornadoes and fires at the wildland-urban interface.

I visited New York City frequently and spent a considerable amount of time there during the investigation, and continue to often visit the city. Our team received excellent, vitally needed support and cooperation from New Yorkers. New Yorkers are extraordinarily resilient. Just look at how they responded to the tragic events of 9/11. It is gratifying to see all the great progress that has been made in fully rebuilding the World Trade Center complex and the many new safety features that have been implemented consistent with our recommendations.

In 2014, my family and I had the privilege of visiting the largely underground 9/11 Memorial and Museum as well as the newly built 104-story One World Trade Center. The curators who were planning the 9/11 museum came to NIST several times to look at pieces of World Trade Center steel that were under investigation at the time. The curators ended up selecting pieces of steel that are now in the museum. They include two large exterior columns that were struck by the aircraft when it crashed into WTC 1. These visits were emotional experiences for me since they brought back vivid memories of the tragic events and consequences of that day and how far we have come as a nation since then. I am very proud of what the NIST team accomplished through our investigation and the lasting impact of the resulting changes on the safety and resilience of our nation’s buildings, their occupants, and emergency responders.

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. Visit

About the author

S. Shyam Sunder

S. Shyam Sunder led the Federal Building and Fire Safety Investigation of the World Trade Center Disaster in the aftermath of the terrorist attacks of Sept. 11, 2001. He is now the director of NIST’s Special Programs Office and its Chief Data Officer. Sunder is a recipient of the Presidential Rank Award of Distinguished Executive (2017), and the Gold Medal Award (2005) from the U.S. Department of Commerce, its highest honor, for distinguished leadership of the WTC investigation. Sunder was elected to the National Academy of Construction in 2012.

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Well done. Thank you for sharing, and for all your hard work to help others.

Did your team investigate whether toxic fumes were emitted by the burning of insulation on telecommunications wiring?

Did you test for the vibrational properties of the non- metallic components of the structures?
The reason being that if there have been other heavy construction around those buildings before the collapse and the materials were not designed to withstand the vibrations from the nearby heavy construction machinery then there is the possibility of internal cracks in the non-metallic components . The plane hitting it just shook the material to fracture.
I believe the same phenomenom was possible for the surfside collapse in Florida.
There the need to redesign our cement and mortar.

Hi Mr Sunder
I remember watching in horror as the tragic scene was displayed on TV, fortunately being in SA I was not impacted directly but can only imagine the terrible tragic especially after watching Netflix film recently with Nicholas Cage trapped underground. My question to you as a Force Metrologist and seeing various rumours or videos that their was more involved then what meets the eye, as in the speculation that the Towers could not simply collapse straight down and conspiracy theory's that the structural foundations where tampered with or even explosions set of at the precise time to weaken this , i would really like to know from someone like yourself is this was just propaganda or reality that everyone has ignored. I could not imagine where it to happen in Cape Town as it would be like a surreal nightmare and my heart goes out to everyone involved and families who lost there loved ones in the brave call of duty.


In reflecting back on my time at NIST, I often think of the work of you and your team on the WTC investigation as one of the most, if not the most, proud moments of just being affiliated with the Institute and the work carried out there. Your leadership was instrumental in the success of this monumental project. It was a very real and publicly tangible example of the important work done by NIST, which is often lost on the general public. And as you point out, it is less about simply understanding why this disaster happen in the manner that it did, but more importantly, what can we learn to improve our future developments and the safety and, in this case, resilience of existing and future building structures. That's where the real, true, and ironically immeasurable, impact of of NIST's work resides: in the disasters that didn't happen in the future.

Kevin Carr, former MEP Director


Thank you so much for your comments. As you so rightly observe, the real, true, and immeasurable impact of NIST's work on future developments and safety. As you know, this was a team effort with extensive engagement of all those affected by the horrific events and their consequences as well as the broader public.

Shyam Sunder, Lead Investigator

I noticed this article makes no mention of Architects and Engineers for 9/11 Truth. Will NIST disclose to the public the computer inputs of its mathematical models used for simulating how the buildings were destroyed? Thank you.

Henry, Thank you for your question. This information was exempt from public disclosure under Section 7d of the National Construction Safety Team Act because it was determined by the Director of NIST that release of the files might jeopardize public safety. The withheld information contains detailed connection models that have been validated against actual events, and therefore, provide tools that could be used to predict the collapse of a building. The information contained in the withheld files is sufficiently detailed that it might be used to develop plans to destroy other, similarly constructed, buildings.

In Michael Quick v. United States Department of Commerce, National Institute of Standards and Technology, Civil Action No. 09-02064 (CKK) U.S. District Court of the District of Columbia, Apr. 7, 2011, the court upheld NIST’s finding to withhold this information.

Thank you.

Thank you for your response, Mark. Is there a more complete version of the WTC 7 collapse simulation? The only simulation I have been able to review is the NIST WTC 7 collapse model where the building begins to bend and warp in on itself, but the simulation does not show the free fall speed as observed by witnesses and recorded on multiple news footage videos. Would NIST be willing to provide data inputs and mathematical models for just the free fall portion of the collapse after collapse initiation? I believe the collapse initiation is what is being guarded from the public, correct? Thanks again.

Has NIST reached out to Richard Gage and the over 3400 architects and engineers petitioning for a new investigation for the collapse of the buildings? At least one of those members is a former NIST employee, Peter Michael Ketcham. They are willing to share their results after years of investigating the building destruction themselves and using data NIST has provided. Richard Gage does a frequent review of the WTC tower destruction with detailed information on YouTube - ae911truth. Please feel free to publicly refute those sessions as Mr. Gage's presentations seem to refute the each of NIST's findings. It seems unscientific and a bit shady to not address alternative destruction theories. The NIST FAQs seem to be in damage control mode rather than addressing some legitimate concerns that should be reviewed.

But NIST's original findings are incorrect and have been shown by multiple institutions to be incomplete and error prone. You will not provide the input data or mathematical models to prove your findings so that makes NIST suspicious and research illegitimate. Your computer models and conclusions do not match observation. We can clearly see explosions traveling down both towers during the period of destruction. Why will NIST not allow the thousands of engineers, architects, and university professors to provide an alternate theory? We need a new investigation. The hubris for you all to congratulate each other is insulting.

Does the computer model that NIST produced match all the evidence observed? The NIST description seems to be far too complex. What does NIST have to say about all the reported molten steel and the measured temperatures of higher than 2000 F that lasted 6 weeks?

How are you dealing with the active lawsuit against your agency for arbitrary and capricious
actions? You cannot hide from the truth forever. I look forward to the day that a grand jury reviews the mountains of evidence that support WTC1, 2 and 7 were all destroyed via controlled demolition, with thermite as the incendiary.…

Dear Dr Shyam Sunder, I write this message from India. I had the honour to meet you at UL Conference sometime in 2008. The detailed investigation helped for us to also
learn and update our codes back here at India as we revised the National Building Code in 2005 and 2016. We compliment and express gratitude to Team NIST for such extensive research and sharing. With good wishes.

Dear Mr. Sandeep Goel,

We are delighted to learn that NIST's WTC Investigation was helpful in updating the National Building Code of India. Thank you very much for sharing this information with us.

Dear Mr. S. Shyam Sunder et al.,
We all understand how different types of threats and coercion might impact human behavior and professional integrity. Despite all of that, we all should be able, in a time of self-reflection, to realize that truth is a liberating element in life. Especially once we get older and start seeing the world from a different, more mature perspective. We usually start thinking about our legacy and the side of good vs evil we took while inhabiting this planet. It must be very exhausting and "soul" killing to continue walking along the path someone had forced you to walk on a long time ago. It seems to me, that you have become more of a politician over the long years of your career at NIST. Your choice of words in the above article attests to it very clearly. Why don't you look inside of your "heart", think about everything again and do the right thing? You could help to transform this world into a better place for all of us. I can guarantee you that the tangible loses are significantly outweighed by the intangible ones!

Hi Dr Sunder,

I have a question regarding certain footnotes in the NCSTAR 1 report.

Footnotes 13 (Chapter 6 Reconstruction of the Collapses 6.1 APPROACH page 82) and 2 (Executive Summary E.3 SUMMARY OF FINDINGS page xxxvii) are similar with both outlining the overall focus of the NIST investigation and noting what the “Probable Collapse Sequence” refers to. The difference lies in the amount of focus, one noting none, the other noting little. To compare below, discarding the reference component and bracketing the different wording, firstly for 13 than 2 within the remaining body of text, highlights the alternate meanings.

The focus of the Investigation (does not actually include) the structural behavior of the tower after the conditions for collapse initiation were reached and collapse became inevitable.

The focus of the Investigation (includes little analysis of) the structural behavior of the tower after the conditions for collapse initiation were reached and collapse became inevitable.

Although both footnotes refer to the same thing, in light of 13 having been positioned within the main body of the report itself, it’s understood NIST only investigated factors leading to the initiation of collapses of the WTC towers, not the collapses themselves.

Taking into consideration:

1) Executive summaries are a concise version of and typically written after a report &
2) You stated in your Sep 2018 lecture (D-RED Speaker Series: How & Why the World Trade Center Collapsed) at Morgan State University, “There were two people who read every word in those reports, one was me the other was the lawyer.”

Why did you and the lawyer you mentioned choose to publish NCSTAR 1 with an alternate version of footnote 13 in the executive summary, otherwise known as footnote 2?

Hi Dr. Sunder,

I hope you’re well.

I have a question regarding Table P-1. Federal building and fire safety investigation of the WTC disaster as published in NCSTAR 1.

Considering the final report was written with the public in mind as a summary of what happened and how NIST addressed the issues with the sense that most members of the public likely won’t read any of the companion reports, why didn’t NIST detail that no steel was recovered from WTC 7 in the project purpose for the technical area of the Mechanical and Metallurgical Analysis of Structural Steel?

Dear Dr. Sunder, You remark justly that heat weakens steel and make a strong argument for collapse initiation. However, the steel of the as yet undestructed building was intact and not heated from kerosene doused office furniture fires. From the collapse time of both towers we can see a fairly constant acceleration at about two thirds of full free fall acceleration. This shows that the not-yet-collapsing part of the building must have been exerting a force of about one third of the weight of the collapsing top to lower its downwards acceleration. Newton’s third Law then tells us that the force that the collapsing top exerted on the as-yet-undestructed bottom part of the building must have also been a third of the collapsing top’s weight. Given that the towers were engineered to carry five times the weight of everything above at every floor, we have to wonder how it didn’t hold only one third of that weight. However, your team decided to stop investigating building performance at the point of collapse initiation, remarking global collapse became inevitable at that point. Given my argument, an investigation into the building performance during collapse would be worthwhile at some point into the future.

I'm curious to know how experts are able to tell that the two columns were from where the plane struck WTC 1.

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