President George Bush's former counter-terrorism adviser stunned members of the 9/11 commission into silence yesterday when he asserted: "By invading Iraq, the president of the United States has gravely undermined the war on terrorism."
It was the most dramatic moment in an extraordinary day of sworn testimony on Capitol Hill where the 10-member bipartisan commission is hearing evidence from top officials on events leading to 9/11.
Mr Richard Clarke, who has retired after serving four presidents, testified that the Clinton administration had "no higher priority" than combating terrorism, but before the attacks, the Bush administration treated it as "important" but not "urgent".
His testimony is potentially devastating for President Bush, who is fighting for re-election on the basis of his war on terror and Mr Clarke was sharply questioned by Republican members about his pointed criticism of Bush officials in a book published on Monday.
He was accused of acting in a partisan way by former Navy Secretary Mr John Lehman, who said he had a "credibility problem" because he had praised the administration's policies to the media in August 2002.
Mr Clarke said that he was a registered Republican and that as a government official he had the option to resign or obey his instructions to put a positive spin on White House policy. "I've done it for several presidents," he said to laughter.
The silver-haired former aide was applauded by the audience, many of them relatives of 9/11 victims, for making an apology for failing to stop the al-Qaeda attacks, the only top aide to do so.
His testimony yesterday was also potentially damaging for Mr Bush's National Security Adviser, Dr Condoleezza Rice, who has refused to appear under oath.
Mr Clarke testified that Ms Rice had relegated terrorism to a "cluster" of issues when she took office and downgraded his reports to meetings of deputies rather than cabinet members. She had also delayed recommending a new strategy until a meeting on September 4th, 2001, at which Mr Clarke asked "could they imagine a day after hundreds of Americans lay dead and ask themselves what else they could have done."
He said he did not understand why the measures were not taken in February. Asked if Dr Rice's statement describing pre-9/11 anti-terrorism strategy was one of marshalling all elements of national power, including military options, he snapped, "Absolutely not."
He also found "absolutely incomprehensible" the FBI's failure to tell the White House before 9/11 that al-Qaeda suspects were in the US and one had flight training: if he had known he might have connected the dots, he said.
Mr Clarke said he had offered to resign out of frustration in the summer of 2001 and was to be reassigned to focus on cyberspace terrorism. Deputy Secretary of State Mr Richard Armitage appeared for Dr Rice later and said that there had been "stunning continuity" from the Clinton to the Bush administration on anti-terrorism policy.