The United States Congress has begun hearings into possible US intelligence failures in the run-up to the attacks of 11 September.

There has been a series of recent revelations suggesting that clues were missed prior to the attacks, and frequent leaks about an apparent US intelligence failure.

The Senate and House of Representatives Intelligence Panels are meeting behind closed doors to investigate the roles of the various agencies involved.

In the first session on Tuesday, members observed a minute's silence for the victims of the attacks, before adopting an "initial scope of inquiry".

We need to be aggressive and rigorous in this inquiry, asking the right questions like who knew what?

Senator Barbara Mikulski

They will now attempt to establish what the FBI, CIA and other agencies knew and when, what they did about it, what they have learned since the attacks and what can be done to guard against further attacks.

The hearings are secret in order to protect sources, but committee members appear intent on getting answers.

"We need to be aggressive and rigorous in this inquiry, asking the right questions like who knew what?" said Senator Barbara Mikulski.

"And if they didn't know it, why? And what did they do with the information they had?"

'Must do better'

On Thursday, the FBI agent-turned-whistle-blower, Colleen Rowley, is expected to publicly tell a Senate committee why she thinks her bosses hindered enquiries into the man now suspected of being the would-be 20th hijacker.

Even President Bush agrees mistakes were made

The BBC's Nick Childs in Washington says this is more than just political recrimination based on hindsight - everyone from President George W Bush downwards now agrees that mistakes were made and that the agencies must do better in the future.

"In terms of whether the FBI and CIA communicated properly, I think it's clear that they weren't, and now we're addressing that issue," he said at the National Security Agency hours before the enquiry.

"I see no evidence today that said this could have prevented the attacks."

The FBI has borne the brunt of criticism so far, our correspondent says, but the CIA and the National Security Agency have come in for blame too.

The FBI has announced a massive reorganisation.

However, part of the Congressional enquiry will be into whether these changes are in fact the right ones.