The Conspiracy Files delves into the final mystery of 9/11: a third tower at the World Trade Centre, which along with the Twin Towers, also collapsed that day. But this skyscraper was never hit by a plane.
The 47-storey tower collapsed seven hours after the Twin Towers and it has become the subject of heated speculation and a host of conspiracy theories which suggest it was brought down by a controlled demolition.
Nearly seven years on, the final official report on the World Trade Centre is due to be published in July.
Official investigators are expected to conclude that fire caused the collapse of this third tower at the World Trade Centre. But that makes this the first and only skyscraper in the world to collapse solely due to fire.
The Conspiracy Files explores many unanswered questions to try to find out what really happened, and why some people think there was a sinister plot to destroy the building.
Produced and Directed by Mike Rudin
Assistant Producer: James Giles