The president and Vice President Dick Cheney met with the panel in private, without taking oaths, and with no recording or transcript made. His counsel and two other lawyers were also present.
"It was wide-ranging. It was important," Mr. Bush said in brief remarks outside the White House, after emerging from the meeting. "It was just a good discussion and I appreciate the members."
"They had a lot of good questions and I'm glad I did it. I'm glad I took the time," the president said. Asked if he declined to answer any questions, the president replied, "I answered every question they asked."
Mr. Bush refused to get into the details of what he was asked, or what he answered. Cheney did not join Mr. Bush in the appearance before reporters. Asked why he and Cheney appeared together, the president said if they had anything to hide, they would not have appeared at all.
In a statement, the Sept. 11 commission said the president and vice president were "forthcoming and candid."
"The meeting was extraordinary and lasted for more than three hours," the statement read. "The information they provided will be of great assistance to the commission as it completes its final report."
The White House initially opposed the creation of the panel investigating the Sept. 11, 2001, attacks that killed some 3,000 people in New York City, Washington and Pennsylvania. Mr. Bush and Cheney agreed to answer questions jointly after sparring with the commissioners for months over ground rules for the meeting.
In the question-and-answer session, Mr. Bush faced a familiar challenge: convincing Americans that he responded appropriately to an intelligence system that CIA Director George Tenet said was "blinking red" with warnings of a terrorist strike.
Except in this case,
The White House took steps to shield the session from public view. It refused to allow reporters or photographers in for any of the session, and would not agree to release pictures taken by official White House photographers.
Ahead of the meeting, the White House this week said Mr. Bush had been "refreshing his memory," reports CBS News Correspondent Peter Maer.
He was preparing by reviewing documents from the months leading up to the attacks and has been consulting with White House chief of staff Andy Card, national security adviser Condoleezza Rice and Gonzales.
The president and vice president were to be quizzed about why, for instance, the Bush administration didn't make terrorism a more urgent priority, especially after an Aug. 6, 2001, presidential daily brief that, among other things, warned of "patterns of suspicious activity in this country consistent with preparations for hijackings."
The Bush administration has said it did make terrorism a top priority, and that there was nothing in the memo that specified the type, time or place of an attack on America.
Another issue Mr. Bush and Cheney are expected to confront is Iraq. Both men have been accused of being distracted by Iraq in the months before Sept. 11, making them inattentive to warnings pointing to a terror attack.
In newly released books (both published by a firm owned by CBS owner, Viacom), Bob Woodward and former White House terrorism coordinator Richard Clarke separately contend that Bush and Cheney were fixated on finding an Iraqi link to the attacks. The administration has denied it.
The president's meeting is not without precedent. Then-President Ford, who testified before Congress in public about his 1974 pardon of President Nixon. President Reagan testified in private before a panel probing his administration's role in the 1980s Iran-Contra affair.
The effect of Mr. Bush and Cheney's highly classified Q&A session with the commissioners might not be known until the panel releases its final report, which is due out this summer, about three months before the fall presidential election.
"It's very important because of the timing, just before the election," said James Thurber, director of American University's Center for Congressional and Presidential Studies. "He (Bush) is very strong in the polls on homeland security, and this may undermine it a little bit."
The political stakes would be even higher, though, if the meeting were televised like the commission's recent hearings, said Thomas Mann, an analyst at the liberal-leaning Brookings Institution.
"The president is basically placing his re-election on the argument that he is the commander in chief of the war on terrorism," Mann said. "Anything that calls that into question is potentially damaging. If it were public, he'd be out there with a message, pre-scripted."
There are two, sharply divided schools of thought on whether or not the president's meeting the panel will satisfy the public. Mr. Bush's supporters see his and Cheney's appearance as enabling them to show maximum cooperation and get on with other business.
"This is a closure in terms of procedures and in terms of some of the symbolism that is associated with the commission," Rich Bond, a former chairman of the Republican National Committee, said.
Critics, however, see it as a reluctant concession after months of resistance.
Lanny Davis, who was special counsel to Mr. Clinton, said it will be hard for Mr. Bush and Cheney to demonstrate full cooperation given their past resistance to the panel and their "insisting on appearing together."