NEW YORK (AP) -- Searching for clues frame by frame, engineers studying the collapse of the World Trade Center have seen the twin towers fall hundreds of times on videotape.
Despite the widely accepted notion that no skyscraper could have survived an attack by a fuel-engorged jet, a federally commissioned team of engineers is trying to determine exactly when and how the 110-story twin towers collapsed.
The engineers leading the federal investigation met for the second time this weekend -- in private, at an undisclosed location -- to discuss their progress. The 23-member team hopes to complete its investigation by April; its findings may resolve some of the unanswered questions from Sept. 11 and provide ways to address future disasters.
The team is particularly interested in footage showing the position of the north tower's 360-foot transmission antenna as the building crumpled at 10:29 a.m., nearly an hour and 45 minutes after American Airlines Flight 11 sliced through the 91st floor.
"They're looking at how vertical the antenna remained. That would give them perhaps some indication as to how the collapse started," said Larry Roth, executive director of the American Society of Civil Engineers, the sponsor of the federal inquiry.
The way the antenna tilted, and how it fell, could indicate where the building gave way.
Such details could allow the team to prove or disprove whether the core columns fell first, dragging each floor with them as they fell, as one theory goes. Another suggests that the exterior columns pulled inward, giving in to floor joists weakened by raging fires.
"I know they're interested in which direction you can see the exterior columns going," said John Durrant, executive director of the organization's Structural Engineering Institute. "Are they going out or into the collapse at particular stages?"
When completed 30 years ago, the towers were built to withstand the impact of a Boeing 707. The unusual tubular design -- a load-bearing interior core and perimeter walls of glass and 62 steel columns per side, with no other vertical support -- was celebrated for opening up more floor space. But some have suggested it also made the towers more vulnerable.
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