An FBI memo from Phoenix warning that several Arabs were suspiciously training at a U.S. aviation school wouldn't have led officials to the Sept. 11 hijackers even if they had followed up the warning with more vigor, FBI Director Robert Mueller said Wednesday.

"Did we discern from that that there was a plot that would have led us to the Sept. 11 [attackers]? No," Mueller told the Senate Judiciary Committee. "Could we have? I rather doubt it. But should we have done more in regard to the Phoenix [memo]? Yes."

The Associated Press reported last week that an FBI agent in Arizona alerted Washington in July that several Arabs were suspiciously training at a U.S. aviation school and urged that agents contact other schools nationwide where Middle Easterners might be studying.

The FBI sent the intelligence to its terrorism experts for analysis and was considering a nationwide canvass of flight schools when the suicide hijackers struck. The bureau had not yet alerted other federal agencies, such as those that handle pilots' licenses or immigration visas, despite a recommendation by the Phoenix office that the information be shared.

"I believe the Phoenix memo is going to come to be one of the most important documents in our national discussion about whether we did enough to protect America from the attacks of Sept. 11," said Sen. Richard Durbin, D-Ill.

Mueller said it was unlikely that the FBI could have investigated all 2,000 aviation academies and their 20,000 students between July and September.

"It was perceived that this would be a monumental undertaking without any specificity as to particular persons," Mueller said.

None of the people being investigated in Arizona were involved in the Sept. 11 attack, Mueller said, although he admitted that one or two might have had connections to terror organizations.

Mueller said he did not believe that heads of the CIA or the FBI ever saw the contents of the Phoenix memo, which Sen. Dianne Feinstein, D-Calif., called "much more consequential" than the briefings she gets as a member of the Senate Intelligence Committee.

"I think that recommendations of the agent are something that we should have more aggressively pursued," Mueller emphasized. "I do not believe that it gave the signpost of that which would happen on Sept. 11."

This was Mueller's first time in front of the committee since September's attacks.

Mueller also said:

— That the Sept. 11 hijackers left no paper trail for law enforcers to pick up on. "We have not yet uncovered a single piece of information, either here or in the treasure-trove of information that has turned up in Afghanistan and elsewhere, that mentioned any aspect of the Sept. 11 plot," Mueller said. "As best as we can determine, the actual hijackers had no computers, no laptops, no storage media of any kind."

— That the bureau still is getting leads on who mailed anthrax to Americans late last year. Contaminated mail killed five people and sickened 13. An anthrax-contaminated letter was discovered last October in an office of Senate Majority Leader Tom Daschle, D-S.D. A second anthrax letter was discovered later addressed to Sen. Patrick Leahy, D-Vt., chairman of the Judiciary Committee.

The investigation "is not in any way stalled," Mueller said. "Every day we receive new leads in regard to additional individuals and we have an ongoing very thorough laboratory investigation undertaken."