NIST researchers do inventory of steel from World Trade Center buildings (Photo courtesy of National Institute of Standards and Technology)

About three months into a two-year investigation of the World Trade Center disaster, officials at the National Institute of Standards and Technology say it's too early to rule out any possible scenarios for what caused the buildings to fall.

At a Dec. 9 briefing, NIST Director Arden Bement said NIST feels more study is needed to determine which of the various hypotheses about the WTC collapses is "most probable." Bement adds, "We have concluded that it's too early to exclude any potential sequence of events between the aircrafts' impact and the collapse of the WTC towers."

NIST has run a variety of tests so far, including analyses on some of the more than 200 pieces of WTC steel it now has. Shyam Sunder, NIST's lead investigator in the WTC probe, says officials have located pieces representing nine of the 12 steel strengths used in the perimeter columns and nine of the 11 strengths used for the spandrel beams.

Sunder says that about 250 chemical analyses indicate that most of the perimeter columns are "higher-strength micro-alloyed steels...or chromium-molybdenum steels that would meet U.S. specifications for heat-resisting steels." Most of the columns were made from steel from Yawata Steel, which is now Nippon Steel.

In addition, Sunder says documents from Laclede Steel, fabricator of the WTC floor trusses, show that steel "routinely met or exceeded the specified strengths."

Sunder says NIST has had good cooperation from the many organizations involved in the WTC, including the Port Authority of New York and New Jersey, Silverstein Properties, insurance companies and New York City agencies. He says that none of the parties has refused to give NIST any data it requested, although some information may have been destroyed in the buildings' collapses.

Bement made a request to the public and the media for photos or video images that could aid NIST's probe. More specifically, NIST is seeking images of WTC 7 and views from the south and west sides of the two WTC towers. Bement says, "In particular, there is a dearth of photos of the south side of WTC 7." That side, some have said, was hit by debris from WTC 1, which may have started the fires that led to WTC 7's collapse." NIST is asking anyone who has or knows of such images to contact the agency at wtc@nist.gov or by fax at (301)975-6122.

Sunder says there may be many such photos or videos that haven't been shown already on television or published by newspapers and magazines. "We're not looking for the spectacular photographs," he says, but anything that sheds light on how the buildings looked during the time from impact to collapse.

In addition, Sunder says that as part of the investigation, NIST plans to do face-to-face interviews of as many as 600 WTC occupants and 150 first responders.

Sunder says NIST plans to add outside contractors to supplement its 24-person, in-house WTC team. He says the agency is looking for "world-class experts" and will issue the contract notices "in the coming weeks" on its web site, which is http://wtc.nist.gov.