The FBI had been hunting for two of the suicide hijackers who launched Tuesday's terror attacks for two weeks before they struck, it has been reported.
Newsweek says the FBI started searching for Khalid Al Midhar and Salem Alhamzi in late August after the CIA told them the pair were associated with Osama bin Laden.
Al Midhar had been caught on surveillance tape in Kuala Lumpur meeting a man suspected of involvement in the attack on the USS Cole in Aden harbour, the magazine reports.
Al Midhar and Alhamzi have been named as two of the five hijackers who crashed American Airlines Flight 77 into the Pentagon.
Bin Laden is believed to have masterminded the USS Cole attack, which killed 17 American sailors.
The FBI failed to find the pair before the attacks.
Newsweek quotes an unnamed official as saying: "We had only two weeks and one small piece of information that was clearly bogus about where they were supposed to be."
If true, the Newsweek report will be the first indication that US security services had suspicions about some of the hijackers before the attacks.
The magazine also reports that five of the 19 men named as the hijackers by the US Justice Department may have received training at American military camps in the 1990s.
Their names are similar to those of people who had trained at the Naval Air Station in Pensacola, Florida, the Air War College in Montgomery, Alabama, and Lackland Air Force Base in San Antonio, Texas.