Mr. Clarke, the former National Coordinator for Counterterrorism in the White House, began his testimony before commission members by apologizing to people in the audience who had lost family members in the September 11 terrorist attacks.

"Your government failed you," he said. "Those entrusted with protecting you failed you and I failed you. We tried hard, but that doesn't matter because we failed."

Mr. Clarke served as a top counterterrorism official under four presidents.

He told members of the commission that fighting terrorism was an extraordinarily high priority in the Clinton administration, but was not an urgent concern in the early months of the Bush administration prior to the attacks on New York and Washington.

"I believe the Bush administration in the first eight months considered terrorism an important issue, but not an urgent issue," he said.

Mr. Clarke also criticized President Bush's decision to invade Iraq, saying it has undermined the war on terrorism.

Mr. Clarke's testimony was much anticipated because in a series of interviews and in a recently released book he has alleged that President Bush paid too little attention to the al-Qaida terrorist organization before the September 11 attacks, and then focused too much on a possible link with Iraq.

The White House has mounted a major counter offensive, suggesting Mr. Clarke's criticism in a presidential election year is politically motivated.

White House spokesman Scott McClellan told reporters the former coordinator for counter-terrorism supported President Bush's efforts after the September 11 attacks.

Mr. McClellan says the facts do not back up Mr. Clarke's assertions that the administration ignored the threat from al-Qaida because it was focused on Iraq. "The priority in the immediate aftermath of the September 11th attacks was on Afghanistan and going after the Taleban and al-Qaida," he said.

U.S. Deputy Secretary of State Richard Armitage testified that counterterrorism was an urgent issue for the Bush administration in early 2001. He said in hindsight, however, officials weren't acting fast enough.

In earlier testimony, the Director of the Central Intelligence Agency George Tenet told the panel U.S. intelligence sources lit up in the weeks before the terrorist attacks on New York and Washington.

The CIA director says, however, the warnings were, as he put it, maddeningly short on actionable details. Mr. Tenet served under both the Clinton and Bush administrations and said both presidents cared deeply about the threat of terrorism.