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

The 47-storey trapezoid-shaped building was north of the World Trade Centre towers, across Vesey St in lower Manhattan.

On September 11, it was set on fire by falling debris from the burning towers, but sceptics 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 determined that the building's demise was 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.