White House National Security Advisor Condoleezza Rice says the Bush administration understood the seriousness of the terrorist threat prior to the September 11, 2001 terrorist attacks on the United States. But in sworn testimony before the commission investigating the 9/11 attacks, she said no one thing could have prevented the tragedy.

Condoleezza Rice says the terrorist threat did not suddenly emerge on September 11, 2001.

She told the commission there were warning signs over the course of two decades, starting with the attack on the Marine barracks in Lebanon in 1983.

"The terrorists were at war with us, but we were not yet at war with them," she said. "For more than 20 years, the terrorist threat gathered, and America's response across several administrations of both parties was insufficient."

She says the United States was just not ready, and did not have the intelligence infrastructure in place to deal with the terrorist threat. She says it is wrong to assign blame for September 11, and acknowledges no one thing could have kept the attacks from occurring.

"As your hearings have shown, there was no silver bullet that could have prevented the 9/11 attacks," said Ms. Rice.

Speaking in a packed hearing room and before a national television audience, Ms. Rice stressed President Bush always took the terrorist threat seriously. She pointed to a series of briefings in the early months of the administration, and her decision to keep on terrorism experts from the outgoing Clinton administration to help with the formulation of policy.

"We also moved to develop a new and comprehensive strategy to try and eliminate the al-Qaida terrorist network," she said. "President Bush understood the threat, and he understood its importance. 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.'"

That comment resulted in a testy exchange with commission member Bob Kerry. He said the Bush White House should have taken action much sooner, particularly in response to the 2000 attack on an American Navy destroyer, the USS Cole.

"Especially after the attack on the Cole of the 12 of October, it would not have been a swatting of flies," he said. "We did not need to wait to get a strategic plan."

On numerous occasions during the hearing, Ms. Rice found herself on the defensive. Democrats on the commission, like Mr. Kerry, were aggressive in their questioning and clashed repeatedly with the president's national security advisor. The Republicans were far more subdued, and their exchanges with Ms. Rice could best be described as business-like.

All were aware of the high stakes involved, as the independent commission seeks answers during a presidential election year, when the handling of the war on terror is a big issue.

Ms. Rice, originally, was barred from testifying publicly by the White House in large part because of the confidential nature of the advice she gives the president. All that changed when Richard Clarke, the administration's former counter-terrorism coordinator, went before the panel and alleged President Bush did not give enough attention to the al-Qaida threat prior to the September 11 attacks and was fixated on Iraq.

Condoleezza Rice never criticized him directly, though she disputed his allegations. She forcefully rejected the notion that when the attacks occurred, the president was only interested in Iraq.

"I can tell you that when he went around the table and asked his advisers what he should do, not a single one of his principal advisors advised doing anything against Iraq," said Ms. Rice.

In all, Ms. Rice appeared before the commission for three hours, most of it spent answering questions from the ten commissioners. A date has been set for the panel to hold a private joint session with President Bush and Vice President Cheney, but it has not been formally announced.