In the video aired on Wednesday, they can be seen undergoing additional scrutiny after setting off metal detectors, then being permitted to continue to their gate.

First described in January by the commission investigating the September 11 attacks, the videotape was broadcast publicly for the first time on the eve of the publication of the commission's final report highlighting the failure of US intelligence to prevent the attacks but sparing the White House of any criticism.

The video showed Majid Muqid, Khalid Almihdhar, Hani Hanjur and brothers Nawaf and Salam Alhazmi arriving at the checkpoints.

Moquid and Almidhar first arrived at 7:18am and promptly set off an alarm, said CNN citing a September 11 commission staff report. Alhidhar cleared a second magnetometer, but Muqid did not and was scanned manually with a metal detector.

They were both cleared for boarding. CNN said they allegedly had small knives on them which set off the alarms - pocket knives were not banned aboard passenger planes before the September 11 attacks.

Hanjour arrived at the airport at 7:35am and proceeded through the security checkpoint with no problems, as did Salam Alhazmi one minute later.

His brother Nawaf, however, set off the alarm twice and was scanned manually and cleared for boarding, CNN said.