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.
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The World Trade Center Demolition and the So-Called War on Terrorism
9/11 Myths and Reality
Stranger Than Fiction