By Kevin Anderson BBC News Online Washington correspondent

As potentially explosive hearings begin in the United States Congress to assess intelligence failures ahead of the 11 September attacks, the US intelligence agencies are passing the blame.

For much of the past few weeks, the FBI has been the focus of finger pointing, but now the CIA is also under scrutiny.

FBI Director Robert Mueller will be called on to defend his actions and his agency

It has set off a rear-guard action of leak and counter-leak.

The FBI has been under the media microscope for much of the past few weeks with revelations that the bureau failed to act on a memo from an agent in Arizona who wanted to investigate why several Arab men had enrolled at flying schools.

Agent Coleen Rowley wrote a 13-page, 6,000-word memo to headquarters complaining about its refusal to allow a search of a laptop computer belonging to Zacarias Moussaoui, the man subsequently accused of being involved in the 11 September plot.

In the memo, she accuses FBI director Robert Mueller of failing to admit the bureau's mistakes in the Moussaoui case.

Both Mr Mueller and Ms Rowley will testify before the Senate Judiciary Committee on Thursday.

CIA on the defensive

But on the eve of the hearings, the CIA found itself on the defensive.

Over the weekend, reports said the CIA had failed to share crucial information with the FBI. Early in 2000, the reports said, the CIA knew that two of the future hijackers, Khalid Almihdhar and Nawaf Alhazmi, had met suspected members of al-Qaeda.

But a CIA official responded by saying that the agency had e-mailed information to the FBI, and agents working in the CIA's counter-terrorism centre, in January 2000.

The two men were added to watch lists, but only weeks before the attacks, too late to prevent them from entering the US.

Senator Richard Shelby, the senior Republican on the Senate Intelligence Committee, has emerged as an especially vocal critic of the CIA.

He accused the agency of massive intelligence failures and said that CIA Director George Tenet was "in denial", adding: "But I believe he is totally wrong."

Election calculations

The political manoeuvring comes as closed-door hearings rooms begin on Capitol Hill. The hearings will be held in special secure, soundproof rooms.

The intelligence committees of both the House of Representatives and the Senate will be briefed by their staff on the status of their investigation.

The hearings will not be held in open session until late in June. Both Mr Mueller and Mr Tenet are expected to testify at the public hearings.

But Democrats, sensing the possibility to gain political advantage during this critical election year, are calling for an independent commission to conduct an investigation into intelligence failings.

In the meantime, the CIA, the FBI and other intelligence agencies will seek to limit political damage as Congress considers reforms and assesses blame in the wake of last year's attacks.