In an astonishing twist in the highly-charged investigation, Mr Ashcroft released a secret 1995 memorandum authored by one of the 10 commissioners, former deputy attorney-general Jamie Gorelick, that explicitly restricted FBI agents investigating al-Qaeda's links with foreign governments from sharing what they found with criminal investigators.

He charged that this "wall" between intelligence and criminal investigations of terrorism was "the single greatest structural cause for September 11", and that it was largely created by officials from the Democratic administration of President Bill Clinton.

Mr Ashcroft's attack was the most explicit effort yet by the administration of President George W. Bush to shift blame for the attacks to the Democrats. It came as the commission released two new reports that focused on long-standing problems in the FBI.

The reports also indicated Mr Ashcroft did not make terrorism a high priority before September 11. It quoted the former acting director of the FBI, Thomas Pickard, who said Mr Ashcroft was not interested in briefings on terrorist threats and rejected his request for more funds in 2001.

But Mr Ashcroft tried to move the focus to the previous administration, saying FBI agents were "isolated by government-imposed walls, handcuffed by government-imposed restrictions and starved for basic information technology". A 2002 congressional investigation into the September 11 attacks revealed that one of the biggest problems facing the FBI in terrorism investigations was the "wall" that barred sharing of information between criminal investigators and intelligence-gatherers. Those restrictions prevented quick actions in the cases of several al-Qaeda suspects prior to the September 11 attacks, including Zacarias Moussaoui.

While many of the restrictions date back to the 1970s, Mr Ashcroft charged that the Clinton administration itself was the chief architect.

The 1995 memo written by Ms Gorelick, who was deputy to then attorney-general Janet Reno, who also testified on Tuesday, concerned the launch of an investigation following the 1993 attempt by al-Qaeda to bomb the World Trade Center in New York. It said any information gathered by intelligence investigators should be classified and not provided to criminal agents, and that those in charge of the intelligence investigations should be "walled off" from the criminal investigation.

Ms Gorelick said in a TV interview that the policy was upheld by Mr Ashcroft's department in an August 6 2001 memo.

Mr Ashcroft also charged that the Clinton administration had failed to implement several of the recommendations of its own White House National Security Council after the failed "millennium plot" by al-Qaeda to attack the US. In particular, Mr Clinton did not approve tougher visa and border controls and a round-up of US suspects on immigrations violations or other minor criminal charges.

Mr Ashcroft authorised all those tactics after September 11, as well pushing through Congress the Patriot Act, which eliminated the wall between criminal and intelligence probes of terrorism.