Scheduled to testify Tuesday were Secretary of State Colin Powell and Defense Secretary Donald H. Rumsfeld, as well as their counterparts in the Clinton administration, William Cohen and Madeleine Albright. They are appearing as part of the panel's review of failures in diplomatic and military strategy.
"My feeling is a whole number of circumstances, had they been different, might have prevented 9/11," panel chairman Thomas Kean, the former Republican New Jersey governor, told the CBS News Early Show. They involve everything from how people got into the country to failures in the intelligence system. There's a whole series of things."
Commissioners say questions for Clinton officials include why the administration reverted to a nonmilitary approach despite knowledge that al Qaeda terrorists had planned attacks to coincide with the Dec. 31, 1999, millennium festivities and particularly after the October 2000 bombing of the USS Cole in Yemen.
Officials in the Bush administration, meanwhile, should expect scrutiny about their counterterrorism strategy after taking office in January 2001 and whether officials downplayed the al Qaeda threat despite warnings from Clinton officials as well as growing intelligence chatter about a possible strike during the summer of 2001.
"The next two days are unlike any two days I can remember in Washington, so far as the number of power players appearing in a very short period of time," said former congressman and commission co-chairman Lee Hamilton. "I think it would be a mistake for Gov. Kean or myself or members of the commission to try to prejudge the situation."
The hearing comes as Mr. Bush's re-election campaign is showcasing his role as a wartime president — and it occurs as the White House mounts a furious counterattack to charges from former counterterrorism czar Richard Clarke.
In his book, "Against All Enemies," Clarke wrote that the current president "launched an unnecessary and costly war in Iraq that strengthened the fundamentalist, radical Islamic terrorist movement worldwide."
The book is published by Free Press, a subsidiary of Simon & Schuster. Both CBSNews.com and Simon & Schuster are units of Viacom.
In it, Clarke claims Bush administration officialsand pressed to find a link between Iraq and terrorism.
Clarke said he warned Bush officials in a January 2001 memo, just as they were taking office, about the growing al Qaeda threat after the Cole attack but was put off by national security adviser Condoleezza Rice, who "gave me the impression she had never heard the term before."
"The president needed more," Rice said. "He needed a strategy for al Qaeda that was going to eliminate al Qaeda."
Clarke told CBS News' 60 Minutes that immediately after the attacks on Sept. 11, the administration
"Well, Rumsfeld was saying that we needed to bomb Iraq. And — and we all said, 'But no, no. Al Qaeda is in Afghanistan. We need to bomb Afghanistan.' And Rumsfeld said, "There aren't any good targets in Afghanistan and there are lots of good targets in Iraq."
Rumsfeld told reporters that the administration had discussed early military action against Iraq, but not in the context of the Sept. 11 attacks. He said the concern was Iraqi attempts to shoot down U.S. jets patrolling the no-fly zones.
The White House also
"See if Saddam did this," Mr. Bush is quoted by Clarke as saying.
McClellan said Mr. Bush "doesn't have any recollection" of such a meeting or conversation.
Vice President Dick Cheney said Monday of Clarke's assertions, "I fundamentally disagree with his assessment both of recent history, but also in terms of how to deal with the problem" of global terrorism.
Clarke "wasn't in the loop, frankly, on a lot of this stuff," Cheney said in a telephone interview with radio commentator Rush Limbaugh.
Cheney suggested Clarke "may have had a grudge to bear," that he had left the White House after being passed over for a promotion. Clarke resigned his White House job 13 months ago, after holding senior posts under Presidents Reagan and Clinton and the first President Bush.
Clarke was scheduled to testify Wednesday, along with CIA Director George Tenet; Clinton national security adviser Sandy Berger, who was Rice's predecessor, and Deputy Secretary of State Richard Armitage.
Kean told the Early Show the commission has already heard from Clarke for over 20 hours in private.
"We also heard a number of people from the administration," Kean said. "Obviously, there are conflicts here. That's our job is to sort out those conflicts."
The 10-member commission has invited Rice to testify, but she has declined on the advice of the White House, which cited separation of power concerns involving its staff appearing before a legislative body.
Rice did meet privately with commission members for four hours on Feb. 7 but some commission members and several Democratic senators have said Rice, who frequently appears on television talk shows defending the president's policies, should appear before the commission in public.
Mr. Bush and Cheney have agreed to private, separate meetings with the commission's chairman and vice chairman.
"We would like them to appear before the whole commission. We think that's right and we think that's proper," Kean told the Early Show, adding that in their private session, Mr. Bush and Cheney are "going to answer every question we have."