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Taking Measure

Just a Standard Blog

Putting Together the Big Picture for the World Trade Center Disaster Investigation

collage of images of the 9/11 disaster, some are shaped like puzzle pieces, there is a representation of frames of film
Credit: N. Hanacek/NIST

Imagine assembling a jigsaw puzzle of more than 14,000 pieces without an image on the box showing what the final picture will look like. Imagine that important pieces were missing and needed to be searched for. And imagine that this puzzle was four-dimensional, involving time as well as space.

This was the task for our small dedicated team in the wake of the World Trade Center (WTC) disaster on Sept. 11, 2001. In the weeks following the disaster, my colleagues in the Fire Research Division and I started to think about how we might assemble the story of what had happened. Some of our puzzle pieces were images captured by news photographers, reporters from networks and local television stations, and first responders; but others came from everyday people who — witnessing an unspeakable horror — raised a camera they just happened to have on hand that day. The images were from near and far away; of one building or both; at every angle imaginable; and from the moments after the first plane strike through the collapse of the towers. They were taken with professional-grade equipment and bargain-store cameras alike. Individually, they held isolated scraps of information, but we recognized that together they stored great knowledge.

We needed to be able to organize this material in such a way that the stills and clips could quickly and easily be called up to answer questions. NIST researchers would want to know such things as where and when fire or smoke was visible inside specific parts of the buildings, what time particular windows were broken, and which areas appeared to resist fire spread.

By the time NIST was officially assigned the responsibility for an investigation into the collapse of the WTC towers, we had already selected a digital asset management software system, thought out how a database might be used by researchers, developed search terms, and started to populate that database with early sets of photographs.

Visual materials started to pour in due to the dedicated work of Bill Pitts, who led the NIST image collection effort; Val Junker, a New York City-based photographer who had many contacts within that community; and the scores of professional and amateur photographers who had trained their cameras on the buildings.

To help organize the rapidly expanding collection, we hired students from Montgomery College — a nearby community college — who could devote several hours each week to this task. The students brought their talents and enthusiasm to the project, along with a variety of life experiences. Most, though not all, had a technical background. We discussed the difficulties of the assignment along with the satisfaction of filling in another piece of the puzzle. The work required dedication, and we hope that the students who participated know that we’re grateful for their contributions to the effort.

In order to carry out the task, a protocol was established to systematically incorporate every new photograph or video clip into the database. First, the material was logged in and copied to digital storage media. Videos were cut into continuous segments, with a break in the footage signaling an end to the clip. The most important but challenging task was to determine the time at which each photo and video was filmed. This required careful examination of the images to find some event that could be compared to media within the database whose time had already been established.

For digital cameras, determining the time one photo was taken allowed the times for all other photos from that camera to be established. For consistency, times were fixed relative to the moment of the second plane strike. In the end, we were able to fix the time of 44% of the photographs and 38% of the video clips within plus or minus three seconds. Finally, the database contents were categorized by location, time, viewpoint (near or far), key events such as the plane strike or collapse, the presence of smoke or fire, building damage, and other useful information. As the database grew, gaps were filled in and the overall picture became clearer.

By the time the many thousands of photos and videos that had been collected were incorporated into the visual database, a comprehensive picture of the entire event had been developed. Researchers could use it to get the answers to questions about what the situation on a specific floor was at a specific time.

The assembled puzzle was delivered to NIST researchers working on various aspects of the disaster, from impact to collapse. They supplemented it with information from other sources to enrich their analyses. The images covering the exterior façades, for example, were joined with first responder communications and phone calls from people within the buildings to build a more complete story of what transpired inside the towers.

The database was also used to develop a timeline for all eight sides of the two WTC towers, recording locations of fires and broken windows every few minutes. This was used as both input data and a reality check for the NIST-developed fire model FDS (Fire Dynamics Simulator) that was employed to understand how fire progressed through the buildings. FDS combined data from the building plans of the affected floors and heat flux information from fire experiments performed at NIST in which both intact and damaged WTC office mockups were set ablaze. The visual evidence provided boundary conditions and other information: Each broken window represented a new source of oxygen to feed the flames, while exterior areas that remained without visible fire suggested that interior walls were intact. As the model was moved forward in time, predictions of fire reaching the exterior walls of the building could be checked against the visual evidence; adjustments could then be made to keep the model on track.

Some of the videos were used to estimate the speeds of both planes upon impact. The database documented structural changes in the buildings, including column displacements and sagging floors. One of the videos in the database was used to assess the structural integrity of WTC 2 after impact by comparing the back and forth motions of the building’s windows after the second plane strike to the natural frequencies of the undamaged tower.

NIST’s investigation of the World Trade Center disaster took over three years and the work of more than 200 NIST employees, contractors and non-NIST contributors. Completing our puzzle and delivering the database was just one small part of that effort, but it provided crucial input to an analysis that yielded not only an understanding of what happened on Sept. 11 but information that could be used to prevent or limit the loss of life in future building catastrophes. Recommendations arising from the investigation have led to improvements in national and international building codes, standards and practices.

I have often been asked what it was like to work on this project. My overwhelming feeling is one of gratitude. It was truly a privilege to be able to bring my set of skills to bear on this most difficult event at a time when many Americans felt helpless. This is NIST at its best: providing opportunities for researchers to change the world for the better.

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

Kathryn Butler

Kathryn Butler is a physicist in the NIST Fire Research Division. She is currently working on understanding the fire behavior of fences, mulch and woodpiles in wildland-urban interface (WUI) fires. Butler received a Department of Commerce Gold Medal for her work on the NIST investigation of the World Trade Center disaster and a NIST Bronze Medal for her work on the evacuation of people with mobility impairments during building fires.

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Thanks, This help set our expectation and hopes for Champlain Towers South investigation, and explains the time frame.

Thank you for letting me know that this post has helped you to understand the kind of work that goes into a NIST building failure investigation. I am happy to note that NIST has established a NIST Disaster Data Portal for the public and other stakeholders to upload videos, photos, and other documentation. I expect that this will streamline some of the evidence collection for the Champlain Towers South investigation.

It was a plannned by us government to go to war. No plans could ever enter the wtc.
Be wise and not dumb. I work in aviation for more than 25 and there no way a plane can whether and do that. Twice on top of it.

The stratagem with which they managed to make WTC1-2 and 7 reach the speed of falling of a grave in the void and then the achievement of thermal equilibrium of the site after 100 days, must finally be revealed by your NIST or convince the world that the laws of physics that day in NY did not work.
Here there is a proverb "The devil makes the pots but not the lids" ...

Someone could work in aviation for 25 years, checking the air in tires; or someone could be an aviation engineer. I have never worked in any aspect of aviation, but absolutely understand and know what happened, based upon the science of building construction and building failures. Yes, two planes loaded with fuel can do that, and DID do that.

Anyone capture the picture of MOLTEN STEEL pouring out of the south tower right before it was blown up? How about John Gross ? NISTS " investigation" into all three collapses is a farce !

NIST reported (NIST NCSTAR 1-5A) that just before 9:52 a.m., a bright spot appeared at the top of a window on the 80th floor of WTC 2, four windows removed from the east edge on the north face, followed by the flow of a glowing liquid. This flow lasted approximately four seconds before subsiding. Many such liquid flows were observed from near this location in the seven minutes leading up to the collapse of this tower. There is no evidence of similar molten liquid pouring out from another location in WTC 2 or from anywhere within WTC 1.

Photographs, as well as NIST simulations of the aircraft impact, show large piles of debris in the 80th and 81st floors of WTC 2 near the site where the glowing liquid eventually appeared. Much of this debris came from the aircraft itself and from the office furnishings that the aircraft pushed forward as it tunneled to this far end of the building. Large fires developed on these piles shortly after the aircraft impact and continued to burn in the area until the tower collapsed.

NIST concluded that the source of the molten material was aluminum alloys from the aircraft, since these are known to melt between 475 degrees Celsius (900 degrees Fahrenheit) and 640 degrees Celsius (1,200 degrees Fahrenheit)—depending on the particular alloy—well below the expected temperatures (about 1,000 degrees Celsius or 1,800 degrees Fahrenheit) in the vicinity of the fires. Aluminum is not expected to ignite at normal fire temperatures and there is no visual indication that the material flowing from the tower was burning.

Pure liquid aluminum would be expected to appear silvery. However, the molten metal was very likely mixed with large amounts of hot, partially burned, solid organic materials (e.g., furniture, carpets, partitions and computers) which can display an orange glow, much like logs burning in a fireplace. The apparent color also would have been affected by slag formation on the surface.

"Pure liquid aluminum would be expected to appear silvery. However, the molten metal was very likely mixed with large amounts of hot, partially burned, solid organic materials (e.g., furniture, carpets, partitions and computers) which can display an orange glow, much like logs burning in a fireplace. The apparent color also would have been affected by slag formation on the surface."

That's pure speculation. NIST haven't backed up this bare assertion by physical experiments.

"There is no evidence of similar molten liquid pouring out from another location in WTC 2 or from anywhere within WTC 1."

This is a direct contradiction of the statement made in the same report about NIST's observation in a raw video, depicting molten metal pouring down at 9:27:04 a.m. inside WTC2 on the 78th floor at north face (Cumulus VDB clip: WCBS Dub2 07.avi).

"Closer inspection of Figure 9–32 shows what appears to be a shiny silver string (indicated by a red arrow) passing from the top to the bottom of one of the windows, 78-238, on the 78th floor. In the video from which this image was taken, the material actually appeared to be a liquid pouring intermittently from the
top of the window and splashing at the window base. The liquid looked as if it was glowing. It was only observed during the six seconds that this video recorded this area of the tower. In Section 9.5, a flow of similar material from windows 80-255 and 80-256 on the 80th floor of the north face will be described. It will be hypothesized that the material was most likely aluminum from the aircraft that had melted and was pouring down through an opening from the floor above. This is also a plausible explanation for the current observation. If correct, it suggests that the pile of debris located on the 79th floor above this window may have contained aluminum from the aircraft. Recall that a pile of debris in this area of the 79th floor was present immediately following the aircraft impact. An intense fire had been burning on this debris since the impact that would have provided the heat necessary to melt the aluminum."
[ NIST NCSTAR 1-5A - Chps. 9-Appx. C (Sept. 2005), p. 331 / PDF p. 35]

NIST's visual analysis does not take into account the offset white balance in the footage. The video has an overshot of blue color, therefore this stream of molten metal appear silvery in there.

That's called thermite and it burns much much hotter and longer than any jet fuel. YouTube it. It was used in the world trade center attacks but guess what? It had to be set up in the buildings with the small charges of explosives before the planes hit them. Go check it out

It was not a "disaster" it was a terrorist attack.

Thank you for this article. It is well written. I will certainly never forget the work we did for this investigation or your leadership, Dr. Butler.

Thank you, Rochelle. We couldn't have done it without your help. I'm so proud of the work we did together.

From Mrs. Butler's blog post:
"NIST’s investigation of the World Trade Center disaster took over three years..."

That's not accurate when you include the WTC7 aspect. The entire NIST investigation into the WTC disaster took nearly 6 years and 3 months (officially: August 21, 2002 - November 20, 2008).

Thank you Ms. Butler for this very informative article. Truly sad, however, that we still have conspiracy loons running around 20 years on.

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