The five hijackers who seized Flight 77 can be seen on a newly released video passing smoothly through security checks.
Small knives would not necessarily have been seized
The surveillance video, recorded at Washington Dulles International Airport and made public this week, shows the five attackers who crashed a plane into the Pentagon, killing 184 people as well as themselves.
It shows that security staff submitted four of them to secondary checks after they set off alarms, yet all five were allowed to join the flight.
A lawyer for relatives of the victims argues that the video - the only known footage of one of the four hijack teams boarding their flight - suggests that the hijackers' luggage was not searched, in breach of security procedures.
Two of the Flight 77 hijackers were suspected of al-Qaeda links and had been placed on a US terrorism watch list less than three weeks before 9/11
Four of the hijackers set off alarms
Details are difficult to pick out in the grainy footage from the airport's metal detector area, but it does appear possible to establish some of the checks which the hijackers managed to pass through.
The five hijackers have been identified by the FBI as Khalid Al-Mihdar, Majed Moqed, Nawaq al-Hazmi, Salem al-Hazmi and Hani Hanjour.
The wall of the Pentagon has been completely restored
Hanjour, believed to be the hijackers' pilot, alone passed through security without any secondary checks, the video shows.
Mihdar and Moqed, both dressed conservatively in trousers and collared shirts, set off metal detector alarms as they passed through at about 0718 (1118 GMT), less than an hour before take-off.
Moqed set off a second alarm, and a screener manually checked him with a handheld metal detector.
Mihdar had been on a terrorism watch list since 24 August 2001 along with Nawaq al-Hazmi.
America's National Security Agency had known them both to be associated with al-Qaeda since early 1999.
Check for explosives
Moments after Mihdar and Moqed were checked, Hanjour, dressed in dark trousers and a short-sleeved shirt, passed through.
The last two hijackers, the Hazmi brothers, both in trousers and shirts, then approached the security control.
Nawaq Hazmi, described by investigators as the right-hand accomplice of chief hijacker Mohammad Atta, set off two metal-detectors, and a screener manually checked him with a handheld device.
The brothers were kept waiting at a counter while a screener checked Nawaq's carry-on bag with an explosive trace detector.
They were both cleared to board Flight 77.
Ron Motley, a lawyer for victims' families who are suing the airlines and security industry for negligence, said the screeners had failed to search the men properly despite the alarms being activated.
"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," he said.
However, even if screeners had discovered the utility knives which investigators believe the hijackers used for the attack, they might well have let them pass.
At the time, in the days before the Homeland Security Act, knives could legally be carried aboard planes if the blades were less than four inches long and not considered "menacing".
Relatives of victims of the 11 September 2001 reacted strongly to the video.
"It's horrifying to see the ease at which these people... were able to commit the first step of this horrific crime," said Sally Regenhard, whose fire fighter son died in the World Trade Center.
"It's just unbelievable. It really makes the relatives full of anger and outrage at this system that failed us on so many different levels."