NEW YORK - Surveillance tapes and maintenance logs are among the missing evidence as investigators try to figure out why the World Trade Center collapsed, federal officials said Monday. Many documents destroyed in the disaster "are pretty key in carrying out the work," lead investigator Shyam Sunder said.
The 110-story towers collapsed after two hijacked jetliners plowed into the buildings in lower Manhattan on Sept. 11, 2001. Nearly 2,800 people were killed.
Two hypotheses on the cause of the collapse have emerged since the National Institute of Standards and Technology began its $23 million probe three months ago.
Both theories agree the jetliners damaged floor joints and columns inside and outside the buildings. But they vary on whether the fire-weakened columns failed and alone brought down the buildings or whether floor trusses sagged in the intense heat, pulling the columns inward to collapse.
The lost records probably contain vital information that could help answer questions, Sunder said. Investigators are trying to locate copies of many destroyed documents from the building's owners and city agencies.
Also missing are the original contract specifications for the buildings from the early 1970s. Many believed the towers were built to withstand the impact of a Boeing 707 - the largest aircraft at the time, but much smaller than the jets that crashed into the buildings.
Researchers plan to spend two years on the study. They will analyze trade center wreckage, rely on steel manufacturing experts and interview survivors, victims' relatives and rescue workers.
They have also created a database of more than 1,900 photographs shot that day as the towers burned and fell and are asking for more. They are especially interested in photographs showing the south face of 7 World Trade Center, which was not hit by a plane but burned for hours before collapsing.
An earlier federal study directed by a civil engineering group was criticized for not examining evacuation procedures.
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