"Your government failed you, those entrusted with protecting you failed you, and I failed you," Richard Clarke said, turning to family members in the audience.
Clarke, who served as the chief adviser on terrorism in both the Clinton and Bush administrations, said Mr. Bush didn't see al Qaeda as an urgent threat.
Clarke told the bipartisan panel investigating the Sept. 11 attacks that while President Clinton had "no higher priority" than combating terrorism, President Bush made it "an important issue but not an urgent issue."
Clarke said that "although I continued to say it (terrorism) was an urgent problem I don't think it was ever treated that way" by the current administration in advance of the strikes two and a half years ago.
Clarke slid into the witness chair for widely anticipated testimony just days after publishing a to the threat of terrorism. (The book's publisher and CBSNews.com are both owned by Viacom). The White House has sharply criticized the book and against its author.
The white-haired former government official spoke after the commission released a written report saying that confusion about the scope of the CIA's authority to kill Osama bin Laden had hampered efforts to eliminate the man who heads al Qaeda. The result was a continued reliance on local forces in Afghanistan that had scant chance of success, the commission said.
"The commission needs to ask why that strategy remained largely unchanged throughout the period leading up to 9-11," the report said.
But Clarke drew sharp questioning from Republican commissioners, who said his pointed criticism of Bush officials in his book contradicted his praise for the administration's policies as late as fall 2002.
"I hope you resolve that credibility problem, because I hate to see you shoved aside in the presidential campaign as an active partisan trying to shove out a book," said John Lehman, the former Navy secretary who now is chairman of J.F. Lehman & Co., a private equity firm.
Clarke responded that he was not working for Democratic presidential candidate John Kerry and had no political motivations.
"I will not accept any position in the Kerry administration should there be one," he said, adding that he voted Republican in the 2000 election.
The commission's report said that officials from Mr. Clinton's National Security Council had told investigators the CIA had sufficient authority to assassinate al Qaeda members.
But it also said that agency officials, including CIA Director George Tenet "told us they heard a different message. … They believed the only acceptable context for killing bin Laden was a credible capture operation."
Sandy Berger, who served as Mr. Clinton's national security adviser, testified that the former president gave the CIA "every inch of authorization that it asked for" to carry out plans to kill bin Laden.
"If there was any confusion down the ranks, it was never communicated to me nor to the president, and if any additional authority had been requested I am convinced it would have been given immediately," Berger said in nationally televised testimony before the panel.
Tenet, who preceded Berger in the witness chair, was asked about the issue.
"I never went back and said, 'I don't have all the authorities I need,"' he replied.
"If I felt that I had developed access or capability that required dramatically different authorities, I would have gone in and said, 'This is what I have, this is what I think I can do; please give me these authorities,' and I don't doubt that they would have been granted," Tenet said.
The CIA director, whose tenure has spanned both the Clinton and Bush administrations, praised aides to both presidents for their attentiveness to terrorism. "Clearly there was no lack of care or focus in the face of one of the greatest dangers our country has ever faced" after the Bush administration took office, Tenet said.
He also said unambiguously the nation should be prepared for another attack.
"It's coming. They are still going to try and do it, and we need to sort of — men and women here who have lost their families have to know that we've got to do a hell of a lot better," he said. His remarks brought applause from members of victims' families seated in the audience.
The two days of hearings were remarkable by any account.
Secretaries of state and defense from the two administrations testified on Tuesday, followed on the second day by senior officials who served alongside them in a budding era of terrorism that finally struck home two and a half years ago.
Several members of the commission, including its Republican chairman Thomas Kean, complained through the day about the witness who wasn't there – President Bush's national security adviser Condoleeza Rice. She has talked to the panel privately, but the members wanted her public testimony, something the White House refused to allow.