The FBI missed several opportunities to uncover and possibly prevent the 11 September 2001 attacks on the US, a high-level report has found.
FBI agents failed to pick up on clues about the impending attacks
The bureau did not respond to leads, and failed to follow up an employee's theory that al-Qaeda was sending members to US flight schools, it said.
It turned out that is exactly what Osama Bin Laden's network was doing.
The report by the justice department's inspector general said the mistakes amounted to "a significant failure".
The conclusions are similar to others drawn up after investigations into US intelligence performance in the months and years leading up to September 2001.
The report is a year old but is only being released after becoming the subject of a legal battle involving lawyers for Zacarias Moussaoui, who has been convicted of an al-Qaeda plot against the US, but is fighting a possible death penalty.
The justice department's inspector general, Glenn Fine, criticised the FBI for not knowing about the presence of two of the 11 September attackers in the US.
Saudis Nawaf al-Hazmi and Khalid al-Mihdhar were living openly in San Diego in 2000, and had contacts with known FBI terrorist suspects.
Osama Bin Laden sent operatives to train in the US
"The way the FBI handled these matters was a significant failure that hindered the FBI's chances of being able to detect and prevent the September 11 attacks," Mr Fine said.
A lack of communication between the CIA and FBI was blamed for some of the errors that allowed the future hijackers to slip through the net.
The US has made sweeping changes to its intelligence infrastructure following criticism in the wake of the attacks on New York and Washington.
They include the appointment of a director of national intelligence, John Negroponte, to co-ordinate the work of 15 separate agencies, and the creation of a national counter-terrorism centre.
The FBI also said it had taken action to address faults identified in the report.
It said "no terrorism lead goes unaddressed".