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Sunder said his team investigated the possibility that an explosion inside the building brought it down, but found there was no large boom or other noise that would have occurred with such a detonation. Investigators also created a giant computer model of the collapse, based partly on news footage from CBS News, that they say shows internal column failure brought down the building.

Investigators also ruled out the possibility that the collapse was caused by fires from a substantial amount of diesel fuel that was stored in the building, most of it for generators for the city's emergency operations command center.

The 77-page report concluded that the fatal blow to the building came when the 13th floor collapsed, weakening a critical steel support column that led to catastrophic failure.

"When this critical column buckled due to lack of floor supports, it was the first domino in the chain," said Sunder.

The NIST investigators issued more than a dozen building recommendations as a result of their inquiry, most of which repeat earlier recommendations from their investigation into the collapse of the two large towers.

In both instances, investigators concluded that extreme heat caused some steel beams to lose strength, causing further failures throughout the buildings until the entire structure succumbed.

The recommendations include building skyscrapers with stronger connections and framing systems to resist the effects of thermal expansion, and structural systems designed to prevent damage to one part of a building from spreading to other parts.

A spokeswoman for the leaseholder of the World Trade Center, developer Larry Silverstein, praised the government's work.

"Hopefully this thorough report puts to rest the various 9/11 conspiracy theories, which dishonor the men and women who lost their lives on that terrible day," said Silverstein spokeswoman Dara McQuillen.

In discussing the findings, the investigator Sunder acknowledged that some may still not be convinced, but insisted the science behind their findings is "incredibly conclusive."

"The public should really recognize the science is really behind what we have said," he said, adding: "The obvious stares you in the face."