WASHINGTON -- Under sharp questioning, national security adviser Condoleezza Rice insisted Thursday that President Bush fully understood the threat of terrorism before Sept. 11, 2001, but no intelligence foretold the deadliest attack ever on American soil.

Disputing criticism that Bush was negligent, Rice told a national commission "there was no silver bullet that could have prevented" the attacks that killed nearly 3,000 people in New York, Washington and Pennsylvania.

Broadcast live around the world, the hearing turned contentious as Democratic members questioned why alarms didn't ring when Bush was presented with an Aug. 6, 2001, classified memo entitled "Bin Laden determined to attack inside United States."

Former Sen. Bob Kerrey, a Democratic member of the commission, described the memo as saying that "the FBI indicates patterns of suspicious activity in the United States consistent with preparations for hijacking."

Rice dismissed the document as "historical information based on old reporting" and said it did not warn of attacks inside the United States. But she acknowledged it did reveal the FBI had 70 field investigations underway involving al-Qaida in the United States

Commission members unanimously asked the White House to declassify the memo, whose title had not been revealed previously. The White House said it would be declassified -- but not on Thursday.

Relatives of victims killed on Sept. 11 sat in the audience behind Rice, scribbling notes and shaking their heads at times as she rebutted accusations by former counterterrorism aide Richard Clarke that Bush had fumbled opportunities to eliminate al-Qaida.

Unlike Clarke, Rice offered no apology for the government's failure to prevent the attacks.

"Accountability, ma'am, accountability," called out Carie Lemack, whose mother died on the first hijacked plane to hit the World Trade Center. After three hours in the witness chair, Rice shook hands with a few family members and then reached out to embrace a few more.

With much at stake for the president, Rice appeared composed and unruffled even as members challenged her responses and accused her of filibustering with long answers. Rice carried the responsibility of defending Bush's credibility on the issue he has made the cornerstone of his re-election campaign.

Meeting with Clinton

After hearing from Rice, the commission met with former President Clinton for more than three hours. A person familiar with the session said on condition of anonymity that the former president explained the rationale for many of the terror-fighting policies his administration instituted and the message his administration left behind to the incoming Bush administration.

Rice, recalling a rash of vague warnings over the summer, said, "One of the problems here was there really was nothing that looked like it was going to happen inside the United States." She said the threats pointed overseas to possible targets in the Persian Gulf, Israel or perhaps the summit in Genoa, Italy, of leaders of industrialized nations.

Former Republican Gov. Thomas H. Kean of New Jersey, the commission's chairman, agreed later that the attacks probably could not have been stopped.

"There are a number of things that could have been done and had they been done would have been helpful," he said on PBS' "NewsHour With Jim Lehrer." "But in all probability, 9-11 would have happened anyway. Mr. Clarke said the same thing."

Former Democratic Rep. Lee Hamilton of Indiana, the vice chairman, said he wasn't certain. "But there isn't any doubt in my mind that there was a lot of warning coming to policy-makers," Hamilton said.

Rice was pressed on whether she had talked with the president about the existence of al-Qaida cells in the United States after being alerted by Clarke. She said she couldn't recall.

Rice also was challenged on why Bush's national security team met 100 times before it took up the subject of terrorism and whether she bore responsibility for the failure of FBI offices nationwide to be alerted about increased threats before Sept. 11.

After swearing to testify truthfully, Rice sat alone at the witness table, her hands laced in front of her on a red tablecloth as she read a prepared statement.

Rice said the United States, as far back as the Reagan administration more than 20 years ago, mounted an insufficient response to the gathering threat of terrorism. "The terrorists were at war with us but we were not yet at war with them," Rice said.

Historically, democratic societies have been slow to respond to threats, she said, citing provocations before World Wars I and II.

"Tragically, for all the language of war spoken before Sept. 11, this country simply was not on a war footing," Rice said.

Even so, Rice said, Bush "understood the threat and he understood its importance." She said Bush came into office determined to develop a "more robust" policy to combat al-Qaida and told his national security adviser he was "tired of swatting at flies."

Picking up on her testimony, Kerrey noted that Bush failed to order a military strike in response to an attack on the destroyer USS Cole that killed 17 U.S. sailors three months before Bush took office.

"Dr. Rice, we only swatted a fly once ... How the hell could he (Bush) be tired?" Kerrey asked. That was a reference to a 1998 missile strike Clinton ordered against suspected terror training camps.

Former Gov. Jim Thompson, a Republican commission member from Illinois, also expressed unhappiness about Bush's failure to respond to the Cole. "Blowing up our destroyers is an act of war against us, is it not?" he asked.

Rice said the administration decided not to respond "tit for tat" with an inadequate response that that would simply embolden terrorists.

The person familiar with Clinton's testimony said he told the panel he did not order retaliatory military strikes after the USS Cole was bombed in the autumn of 2000 because he could not get "a clear, firm judgment of responsibility" from U.S. intelligence before he left office the following January.

U.S. intelligence determined al-Qaida sponsored the attack only after the Bush administration took office.