Federal investigators believe the second World Trade Center (search) tower fell much more quickly than the first because it faced a more concentrated, intense fire inside, officials said Tuesday.

The detailed hypothesis was discussed at a meeting of investigators with the National Institute of Standards and Technology (search), part of the Commerce Department.

NIST investigators are preparing a report, to be released later this year, detailing how and why the towers collapsed after being struck by fuel-filled jetliners on Sept. 11, 2001.

Lead investigator Dr. Shyam Sunder said Tower 2 collapsed more quickly than Tower 1 because the fire was more concentrated, weakening sections of interior and exterior support columns more quickly.

Tower 1 was struck first and stood for 103 minutes, almost twice as long as Tower 2, which remained standing for only 56 minutes.

"In Tower 2, you had a large concentration of combustible debris in the northeast corner, and the fire there was a more persistent fire," said Sunder.

The flames stayed strong in part because the impact of the plane stripped away much of the fireproofing along the floors, investigators said.

NIST probers now suspect the stripping effect of the collision was far more decisive in the course of the fire than whether individual floors had more or less fireproofing material.

Investigators also say the towers would have probably remained standing were it not for the raging fires inside, which weakened the steel supports. The jet fuel from the planes burned away within minutes, but the office material and the plane debris continued to burn and break down the structural integrity of the buildings.

As the fire continued, the heat and extra stress on the interior support columns caused them to compress downward. When the building's steel skin couldn't handle the extra weight, it began to buckle.

Investigators have singled out an Associated Press photograph that they said may provide evidence to support their theory of how the buildings collapsed. The photo, taken shortly before the collapse of Tower 2, shows a "kink" in the building's corner at the 106th floor.