The head of the CIA has told the 9/11 commission that the US was "in effect, unprotected" when it was attacked.
Mr Tenet said the CIA failed to convert knowledge into action
George Tenet said systemic problems in the intelligence community had left the country vulnerable to the attacks which killed nearly 3,000 people.
The problems would take five years to fix, he told the bipartisan commission investigating the 2001 attacks.
He also said he did not speak to George Bush in the month before the attacks, when Mr Bush was on holiday in Texas.
"He's in Texas and I'm either here [in Washington] or on leave for some of
that time," Mr Tenet said in response to a question from commissioner Tim Roemer. "In this time period, I'm not talking to him, no."
Mr Bush received a briefing on 6 August 2001 saying that Osama Bin Laden was "determined to attack inside the US". The White House took the extremely unusual step of declassifying the memo at the weekend.
But Mr Tenet said he did not tell the president about the arrest that month of Zacarias Moussaoui - who has been charged with involvement in the 11 September plot.
The FBI and the CIA have been criticised for failing to recognise the threat that led to the attack.
Testifying after Mr Tenet, FBI head Robert Mueller told the commission that steps had been taken to integrate the two organisations at virtually every level.
"We can and are fixing what is wrong with the FBI," he said.
In his opening statement to the commission, Mr Tenet said the CIA had made mistakes, and that he "felt a sense of loss that we did not stop those attacks".
"We all understood Bin Laden's attempt to strike the homeland. We never translated this knowledge into an effective defence of the country," he said.
Mr Tenet told the commission that when he took over the CIA in 1997, the intelligence community was in "disarray", with problems including a lack of communication and outmoded technical equipment.
He said budgets were declining despite a possible rise in threats against the United States.
"Human intelligence capabilities were in disarray - we were not hiring new analysts... or giving them the tools they need," he said.
He said the problems were being corrected, but that it would take five years to build the kind of counter-intelligence operations the US needs.
'Who is in charge?'
Mr Tenet gave his evidence in the wake of a commission report suggesting that no one was in overall charge of intelligence in the run-up to the 11 September hijackings.
The report, released on Wednesday, said that the various agencies responsible for intelligence "lacked the incentives to co-operate, collaborate, and share information".
WEDNESDAY'S MAIN WITNESSES
CIA director George Tenet (second appearance)
FBI director Robert Mueller
Terrorism Threat Integration Center director John Brennan
Mr Tenet, who as director of central intelligence was nominally in charge of all the various agencies, "focused his energies on... the CIA, which is a fraction of the nation's intelligence capability", the report said, noting that his predecessors had done the same.
"The question remains: who is in charge of intelligence?" the report demands in conclusion.