US Attorney General John Ashcroft and two former FBI directors will face tough questions later today over why authorities failed to prevent the September 11th, 2001, attacks and how 70 investigations did not uncover the plot.
Mr Ashcroft, former FBI director Louis Freeh and former acting FBI director Thomas Pickard will testify before the national commission examining the performance of intelligence agencies prior to the 2001 attacks that killed about 3,000.
The FBI is likely to argue it was given too few resources to fight terrorism properly; Mr Ashcroft - whose department controls the FBI budget - will defend the allocation of money.
They are expected to face tough questioning, in part about an August 6th, 2001, secret briefing in which the CIA told President Bush - more than a month before the attacks - that authorities knew al-Qaeda members were in the United States.
According to the briefing, the FBI had detected suspicious activity "consistent with preparations for hijackings or other types of attacks." It said the FBI was conducting about 70 separate field investigations related to Osama bin Laden.
A former FBI official said at the time the bureau was investigating every telephone number and any organisation that was known to have any connection with al-Qaeda.