US Attorney General John Ashcroft has robustly defended the actions of the Bush administration prior to the 11 September 2001 attacks.
Attorney General John Ashcroft gave a combative performance
He told the commission looking into the attacks that his determination to fight terrorism had been hampered by the policies of the Clinton leadership.
The US president later joined in the defence of his administration's role.
On Wednesday, the heads of the FBI and CIA will face the commission along with other senior intelligence officials.
They are expected to face questions about the performance of intelligence services since the attacks.
'Destined to fail'
President Bush said on Tuesday that if he had had any inkling that people would fly aeroplanes into buildings on 11 September 2001, he would have "moved heaven and earth" to save the US.
The available intelligence had given no warning of the impending attacks, he said.
Earlier, the attorney general expressed sorrow and anger over the events, and said he would have deployed all available resources to prevent the attacks had he known they were coming.
John Ashcroft said that policies adopted during the previous Clinton presidency had severely handicapped intelligence and law enforcement agencies.
"The simple fact of September 11 is this: we did not know an attack was coming because for nearly a decade, our government had blinded itself to its enemies.
"The old national intelligence system in place on September 11
was destined to fail."
Mr Ashcroft said he had recommended finding and killing al-Qaeda leader Osama Bin Laden after earlier attacks on US interests abroad.
He said he was supported by National Security Adviser Condoleezza Rice, but laws in place at the time hindered any such plan.
Mr Ashcroft said a legal wall had been put in place to separate criminal investigators from intelligence agents.
"Even if they could have penetrated Bin Laden's training camps, they would have needed a battery of lawyers to approve the capture," he said.
He said the passing of controversial national security legislation in the wake of 11 September, such as the Patriot Act, had removed such barriers to action.
On Tuesday, the National Commission on Terrorist Attacks Upon the United States published a preliminary report in which it accused the FBI of failing over several years to respond to the steadily growing threat of terrorism.
It said the FBI was hamstrung by poor intelligence, a failure to share information and insufficient resources.
The report claimed that 66% of the FBI's intelligence analysts were not qualified to do their job and that on the day of the attacks, about 1,300 agents - just 6% of the FBI's total personnel - worked on counter-terrorism.
WEDNESDAY'S MAIN WITNESSES
CIA director George Tenet (second appearance)
FBI director Robert Mueller
The report also notes that Mr Ashcroft rejected an appeal for more funding a day before the attacks.
Former FBI director Louis Freeh, who resigned in June 2001, acknowledged to the commission that FBI counter-terrorism operations were severely under funded and understaffed.
Appearing before the hearing, he said requests made in the three years to 2003 for 1,895 more staff - including Arabic linguists - produced an increase of 76 posts, as terrorism was not then a national priority.