The memo from Mr. Clarke's group in July 2001 about F.B.I. activities adds another piece of evidence to the document trail, but it is unlikely to resolve the questions over whether the administration did enough to deter an attack.

White House officials, who spent several weeks attacking Mr. Clarke's credibility, said Friday that they believed the memo from his counterterrorism group was an accurate reflection of steps the White House took to deter an attack. But they questioned whether the F.B.I. executed the instructions to intensify its scrutiny of terrorist suspects and contacts in the United States.

In April 2001, the F.B.I. did send out a classified memo to its field offices directing agents to ''check with their sources on any information they had relative to terrorism,'' said a senior law enforcement official who spoke on condition of anonymity. But with the level of threat warnings increasing markedly over the next several months, there is no indication that any directive went out in the late June period that was described in the memo from Mr. Clarke's office.

That summer saw a string of alerts by the F.B.I. and other government agencies about the heightened possibility of a terrorist attack, but most counterterrorism officials believed an attack would come in Saudi Arabia, Israel or elsewhere. Many also were worried about a July 4 attack and were relieved when that date passed uneventfully.

For months, the F.B.I. had been consumed by internal problems of its own, including the arrest of an agent, Robert P. Hanssen, on espionage charges, the disappearance of documents in the Oklahoma City bombing case and the fallout over the Wen Ho Lee spy case. Moreover, the bureau was going through a transition in leadership, with its longtime director, Louis J. Freeh, retiring in June 2001. He was replaced by an acting director, Thomas J. Pickard, until the current director, Robert S. Mueller III, took over in September, just days before the deadly hijackings. All three men will testify at next week's commission hearings and are expected to face sharp questioning about whether the F.B.I. did enough to prevent an attack in the weeks and months before Sept. 11.

At this week's appearance by Ms. Rice, several commissioners sharply questioned whether the F.B.I. and the Justice Department had done enough to act on intelligence warnings about an attack.

''We have done thousands of interviews here at the 9/11 commission,'' said Timothy J. Roemer, a Democratic member of the panel. ''We have gone through literally millions of pieces of paper. To date, we have found nobody -- nobody at the F.B.I. who knows anything about a tasking of field offices'' to identify the domestic threat.