In her opening statement, Ms. Rice told commission members that for decades the United States did not respond sufficiently to the rising threat of terrorism.

Ms. Rice testified, however, that before the September 11 attacks the first major national security policy directive developed by the Bush administration was aimed at the elimination of the al-Qaida terrorist network.

"President Bush understood the threat, and he understood its importance," she said. "He made clear to us that he did not want to respond to al-Qaida one attack at a time. He told me he was 'tired of swatting flies.'"

Ms. Rice's testimony follows allegations by former White House counterterrorism chief Richard Clarke, who told the commission last month the Bush administration did not make terrorism an urgent priority until after the September 11 attacks.

"My view was that this administration, while it listened to me, did not either believe me that there was an urgent problem or was unprepared to act as though there were an urgent problem," said Richard Clarke.

Ms. Rice told commissioners there was "troubling" intelligence in the weeks before the attacks, but called it "frustratingly vague."

The national security adviser pointed to structural and legal problems that prevented agencies from working together before Osama bin Laden's al-Qaida terrorist group stuck.

She told the panel there was no silver bullet, or simple solution, that could have prevented the attacks and the nation was "blind" to the disaster before it occurred.

In an unusual public airing of classified information, details of an August 6, 2001 secret intelligence briefing to President Bush were disclosed during the hearing. Under pointed questioning Ms. Rice revealed the title of the briefing was quote: "Bin Laden Determined to Attack Inside the United States."

Commissioner Bob Kerrey said the briefing mentioned concerns by the Federal Bureau of Investigation (FBI) about airline hijackings, and Ms. Rice responded by saying warnings were sent out through the Federal Aviation Administration (FAA).

Kerrey: In the spirit of further declassification, this is what the August 6 memo said to the president. That the FBI indicates patterns of suspicious activity in the United States consistent with preparations for hijacking. That is the language of the memo that was briefed to the president on the 6th of August.

Rice: And that was checked out and steps were taken through FAA circulars to warn of hijackings. But when you cannot tell people where a hijacking might occur, under what circumstances, I can tell you that I think the best antidote to what happened in that regard would have been many years before to think about what you could do, for instance, to harden cockpits. That would have made a difference.

Lee Hamilton

Ms. Rice said intelligence reports of threats in the months before the September attacks, focused primarily on plots overseas, especially in the Middle East.

She told the commission that the briefing "did not raise the possibility that terrorists might use airplanes as missiles."

On September 11, 2001 terrorists hijacked four airplanes and rammed them into the twin towers of the World Trade Center in New York, the Pentagon outside Washington and into a field in Pennsylvania, killing nearly 3,000 people.

After the hearing commission Vice Chairman Lee Hamilton gave Ms. Rice high marks for her testimony.

"Dr. Rice was a very strong witness, very well prepared," he said. "I do not think we asked her any questions that threw her at all. She was very articulate."

Ms. Rice had already spoken to the commission in private, and the White House allowed her to testify in public only after a barrage of criticism.

President Bush and Vice President Dick Cheney are to appear before the panel in private.