A video has emerged showing security checks on one group of 11 September hijackers before they boarded the plane they crashed into the Pentagon.
The video of the hijackers has shaken victims' families
Four men undergo additional checks after setting off metal detectors at Washington's Dulles airport.
On Thursday the commission probing the attacks on New York and Washington is due to make its final report.
The commission is expected to recommend extensive reforms to the United States intelligence services.
It will also list what are described as 10 missed opportunities to detect the plot in the years leading up to the attacks.
Almost 3,000 people died when two hijacked airliners were flown into New York's World Trade Center and one into the Pentagon in Washington, with a fourth crashing into a field in Pennsylvania.
BBC News Online world affairs correspondent Paul Reynolds says the measures proposed in the report would amount to the most significant change since the CIA itself was founded after World War II.
The surveillance video shot at Dulles airport on the morning 11 September shows four of the five hijackers being subjected to additional searches before being allowed onto American Airlines flight 77.
Two of them, Khalid Al-Midhar and Nawaq Alhamzi, had been known to be associated with al-Qaeda since early 1999 by the US National Security Agency.
The two were put on a terrorism watch list just weeks before the attack - on 24 August 2001.
The hijackers were screened with a handheld device, and one man had his bag checked with an explosive trace detector.
A fifth man did not set off any alarms.
Investigators believe that the hijackers were carrying utility knives, which at that time were legal in carry-on baggage.
The crash killed all 64 people on board, and 125 employees at the Pentagon.
The video was obtained by the Associated Press from lawyers Motley Rice, who are representing some survivors' families in their efforts to sue airlines over the 11 September attacks.
"Even after setting off these alarms, the airlines and
security screeners failed to examine the hijackers
baggage... or discover the weapons they would use
in their attack," said lawyer Ron Motley.
Victims' relatives have reacted angrily to the video.
"It's chilling... to see how the ease with which these people were able to commit the first step of this horrific crime," Sally Regenhard, who lost her son in the World Trade Center attacks, told the BBC's World Today programme.
Congress leaders viewed the final report, by the bipartisan National Commission on Terrorist Attacks Upon the United States, on Tuesday. White House aides were due to be shown it on Wednesday.
The report will be presented at 1130EDT in Washington (1530 GMT).
Bush promised to listen to the report's recommendations
It is almost 600 pages long, and follows testimony from more than 1,000 witnesses and examination of as many classified documents.
It will propose a radical shake-up of US intelligence, including the appointment of a new intelligence czar to oversee 15 spy agencies.
According to the Washington Post newspaper, it will also list 10 instances where the government could have unravelled the plot.
Six of these came during the current administration of President George W Bush, the other four under his predecessor Bill Clinton.
The commission is also expected to lay out new claims about possible links between al-Qaeda and Iran.
President Bush is promising to listen to the recommendations, although last month he disputed findings by the commission that there was no link between al-Qaeda and former Iraqi leader Saddam Hussein.
Correspondents say that in the middle of an election campaign the incumbent president has the most to lose from the report.