The Conspiracy Files returns with a new programme about the final mystery of 9/11: a third tower at the World Trade Centre, which along with the Twin Towers, also collapsed that day.
Seven years on, the last official report on the World Trade Centre has finally been published.
Watch a preview of the programme
The documentary updates an edition of The Conspiracy Files shown earlier this year.
It features new interviews with the lead official investigator and an important new eyewitness together with architects, scientists and others who think there was a sinister plot to destroy the building.
World Trade Centre Building 7 has become the subject of heated speculation and a host conspiracy theories suggesting it was brought down by a controlled demolition.
The 47-storey skyscraper was never hit by an plane.
There is little photographic evidence of the building on 9/11.
Yet seven hours after the Twin Towers collapsed, this third huge tower at the World Trade Centre collapsed in a matter of seconds.
Afterwards, the thousands of tonnes of steel from the building were taken away to be melted down in the Far East.
From the outside it was an anonymous building, but it housed some unusual tenants: the CIA, the Secret Service, the Pentagon and the very agency meant to combat terrorist attacks in New York, the Office of Emergency Management.
Some people think WTC 7 had to be demolished because it is where plans were hatched for a massive conspiracy on 9/11.
Official investigators concluded 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 because of fire.
The Conspiracy Files explores the questions that continue to be raised and tries to find out what really happened.
Produced and Directed by Mike Rudin Assistant Producer: James Giles