Federal investigators say they have solved a mystery of the September 11, 2001 attacks, the collapse of World Trade Centre building 7, a source of long-running conspiracy theories.

The 47-story trapezoid-shaped building was located at the north of the World Trade Centre towers, across the Vesey Street in lower Manhattan.

On September 11, it was set on fire by falling debris from the burning towers, but skeptics have long argued that fire and debris alone should not have brought down such a big steel-and-concrete structure.

Scientists with the National Institute of Standards and Technology say their three-year investigation of the collapse determined the demise of WTC 7 was actually the first time in the world a fire caused the total failure of a skyscraper.

"The reason for the collapse of World Trade Centre 7 is no longer a mystery," said Dr Shyam Sunder, the lead investigator on the team.

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 building has been the subject of a wide range of conspiracy theories for the last seven years, partly because the collapse occurred about seven hours after the twin towers came down.

That fueled suspicion that someone intentionally blew up the building in a controlled demolition.

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.