US President George W Bush has said he will now back holding an independent investigation into intelligence failings in the months leading up to the 11 September attacks.

Someday, someone will die, and... the public will not understand why we were not more effective and throwing every resource we had at certain problems

FBI agent's e-mail

Previously the White House had said such an investigation would divert the authorities from fighting terror.

The announcement of the policy reversal comes after new information emerged at a congressional hearing, highlighting poor communications between intelligence agencies.

Specifically it has been revealed that some of the hijackers were known to the Central Intelligence Agency (CIA) for nearly two years before the attacks.

But when an agent from the Federal Bureau of Investigation (FBI) wanted to pursue one of them just weeks before the attacks, permission was denied.

Chilling warning

The BBC's Steve Kingstone says calls for the inquiry have proved impossible to resist in light of what has surfaced when CIA and FBI officials testified anonymously in a joint House-Senate investigation on Friday.

Bush has been under mounting pressure to hold an inquiry

It appears now that at least 20 months before the attacks the CIA was aware of three of the men who would eventually take part in the attacks.

There was intelligence linking them to al-Qaeda, and in January 2000 two of them entered the US.

But the CIA only issued an alert on 23 August, and by then the men were out sight - in the final stages of planning the attacks.

Even after that, an FBI agent in New York was denied permission to pursue one of the men - Khalid al-Mihdar - using his office's full resources.

He said his superiors told him that the matter could not be pursued because the "wall" separating intelligence matters from criminal investigation could not be breached.

"Someday, someone will die, and... the public will not understand why we were not more effective and throwing every resource we had at certain problems," the agent warned in reply in his e-mail.

'Failure piled upon failure'

CIA agents identified Khalid al-Mihdar and Nawaf al-Hazmi as "terrorists" after they attended an al-Qaeda meeting in Malaysia in January 2000.

The inquiry led by Ms Hill studied 400,000 pages of documents

But the agency failed to share the information, and the two men were able to enter the US and live openly using their real names in California.

The men were able to board a major airline flight, and - along with three accomplices - crash the plane into the Pentagon.

Our correspondent says that what is so embarrassing is that the intelligence chain only appears to have been broken after the hijackers arrived in the US.

"Everything that could have gone wrong, did," a CIA officer was quoted by the Associated Press news agency as saying during the congressional hearings.

"This was failure piled upon failure," Democratic senator Carl Levin, said.