The film is a documentary-style re-creation of what Mr Greengrass calls a "believable truth" about what might have happened on the plane and in air-traffic control centres - from the moment a controller hears the first indications of the hijacks to when the Flight 93 passengers storm the cockpit and try to seize control.Unbelievable.
It elapses in real time and is based on dialogue derived from improvisation, giving it a claustrophobic and believable feel. Further blurring the distinction between documentary and drama, several of the characters appear as themselves. Flight 93 pilot Jason Dahl is played by JJ Johnson, a United pilot in real life.
Many of the actors were at the premiere, but Lewis Alsamari, the British-based Iraqi actor who plays hijacker Saeed al-Ghamdi, was refused a visa to enter the US. The American embassy in London gave no reason for its decision. He will see the film for the first time at a private screening in the UK in the next few days.