GAITHERSBURG, Md. — Federal investigators issued a report Thursday concluding that fires brought down a skyscraper next to New York's twin towers on Sept. 11, countering conspiracy theorists who have long believed that explosives somehow caused the collapse.

Scientists with the National Institute of Standards and Technology say their three-year investigation of the collapse of the 47-story World Trade Center 7 was the first known instance of fire causing the total failure of a skyscraper.

The investigators also concluded that the collapse of the nearby towers broke the city water main, leaving the sprinkler system in the bottom half of the building without water.

The structure has been the subject of a wide range of conspiracy theories for the last seven years, partly because the collapse happened about seven hours after the twin towers were felled. That fueled theories that something else might have caused the collapse.

Dr. Shyam Sunder, the lead investigator on the project, said his team investigated the possibility that an explosion inside the building brought it down, but it found there was no large boom or other noise that would have occurred with such a detonation.

Investigators also concluded there was no evidence that the collapse was caused by fires from a substantial amount of diesel fuel that was stored in the building.

The 77-page report concluded that the weakening of a critical steel support column led the rest of the building to fall apart.

"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.