A 'failure of imagination' was the greatest problem that allowed the attacks of Sept. 11, 2001 to happen, the bipartisan commission looking into the attacks concluded.

After 20 months of investigation, the commission released its final report on Thursday, with a number of recommendations aimed at preventing another such attack.

"We were not prepared," said Thomas Kean, the Republican who chaired the independent bipartisan committee.

The nearly 600-page report details a list of intelligence and law-enforcement failures leading up to the attacks, but concludes neither the Bush nor the Clinton administrations could have prevented the attacks.

The panel said the most important failure leading to the attacks was a failure of imagination. "We do not believe leaders understood the gravity of the threat," said the panel.

Kean said the panel found numerous mistakes and failures leading up to the hijackings of Sept. 11, but he said there was no way to determine if steps taken at any point could have stopped the attacks.

"The failures took place over many years and many administrations," Kean said.

Kean did say that much had been done in the wake of the attacks, but the threat remains.

"We believe we are safer today than on 9/11, but we are not safe," Kean said. "We should not expect the danger to recede greatly in the time to come."

Hours before the report was made public Thursday morning, U.S. President George W. Bush called the report's recommendations "constructive" and he said he looked forward to implementing them.

"There is still a threat and we in government have an obligation to do everything in our power to safeguard the American people," Bush told reporters at the White House Rose Garden on Thursday.

Democrat Lee Hamilton, the vice-chairman, said the panel announced a lengthy list of changes. The most urgent is the creation of a national intelligence director to oversee the activities of all intelligence agencies.

The report contains harsh criticism of the way Congress oversees the CIA, the FBI and the remaining 15 intelligence agencies. Many of its recommendations involve changes to that watchdog system.

In a rare news conference, CIA officials defended the organization and rebutted criticisms in the report.. A senior official told reporters that contrary to implications in the report, the agency had regularly reported on threats to civil aviation and told Presidents Clinton and Bush that Osama bin Laden was a danger.

The official, speaking on the condition of anonymity, rejected the charge that the CIA did not share information with the FBI or State Department about two of the hijackers who ultimately crashed into the Pentagon.