Moqed set off a second alarm, and a screener manually checked him with a handheld metal detector.
The pair were known to have travelled together previously and had paid cash to purchase their tickets aboard Flight 77.
Al-Mihdhar and Nawaf al-Hazmi had been known to be associated with al-Qaida since early 1999 by the National Security Agency, and were put on a terrorism watch list on August 24, 2001.
Only Hani Hanjour, believed to have been the hijacker who piloted Flight 77, passed through Dulles security that morning without being subjected to a secondary security check, according to the video.
Moments after Hanjour passed alone through the security checkpoint, wearing dark trousers and a short-sleeved shirt, the final two hijackers, the al-Hazmi brothers, both wearing trousers and Oxford shirts, walked through the checkpoint.
Nawaf al-Hazmi, who was described by investigators as the right-hand accomplice of hijacker- planner Mohammed Atta, set off two metal-detectors, and a screener manually checked him with a handheld device.
Nawaf and his brother, each wearing trousers and Oxford shirts, were directed to a nearby counter, where they appeared to closely examine their tickets while another screener checked Nawaf’s bag with an explosive trace detector. Each was cleared to board Flight 77.
The Associated Press news agency obtained the video from the Motley Rice law firm, which is representing some victims’ families who are suing the airlines and security industry over their actions in the September 11 attacks.
“Even after setting off these alarms, the airlines and security screeners failed to examine the hijackers’ baggage, as required by federal regulations and industry-mandated standards, or discover the weapons they would use in their attack,” lawyer Ron Motley said.
Elaine Teague, who is suing over the death of her 31-year-old daughter, Sandra, said that she had previously been shown the footage by the FBI. However, the terrorists’ faces had been digitally disguised.
Ms Teague said that she was surprised at how relaxed security was, given that airlines had received three warnings from the Federal Aviation Administration.
One such warning, issued in June 2001, cited “unconfirmed reports that American interests may be the target of a terrorist threat from extremist groups”.