Mueller's acknowledgment came amid two new disclosures of what could be missed hints about Sept. 11.
The first was a warning from another agency to the FBI that a Middle Eastern country was seeking to buy commercial flight simulators. The second was a memo from an Oklahoma City FBI agent who reported observing large numbers of Middle Eastern pilots and flight students in his area. Neither memo apparently drew much attention at the time.
"The jury is still out on all of it," Mueller said Wednesday at FBI headquarters. "Looking at it right now, I can't say for sure it would not have, that there wasn't a possibility that we could have come across some lead that would have led us to the hijackers."
On Thursday, Mueller told ABC's "Good Morning America":
"I do not believe, based on what I know now, that we could have prevented the attack. I'm not ruling out the possibility at all. We could have gotten lucky. Absolutely. But from what I've seen now, I do not believe we could have prevented the attack."
Mueller's remarks came after his announcement of a broad reorganization of the FBI, partly because of its failure to predict the attacks.
The FBI's Web site listed the following, in order, as the bureau's new list of priorities:
President Bush endorsed the sweeping changes at the FBI on Thursday, saying the bureau "didn't meet the times" in the run-up to the Sept. 11 terrorist attacks.
"I appreciate Director Mueller's reform measures," said the president.
"The FBI needed to change," he said. "It was an organization full of fine people who loved America but the organization didn't meet the times."
Mr. Bush's support came as Attorney General John Ashcroft announces loosened restrictions on domestic spying, handing the FBI authority to monitor Internet sites and libraries.
"We intend to honor our Constitution and respect the freedoms that we hold so dear" and "we want to make sure we do everything we can to prevent a further attack, to protect America," Mr. Bush said.
The American Civil Liberties Union has criticized the loosening of restrictions on domestic spying, saying they could renew abuses of the past.
The new terrorism guidelines give FBI agents more freedom to investigate terrorism even when they are not pursuing a particular case.
Under existing rules, FBI agents are not allowed to do general research on the Internet or at public libraries unless the information sought directly relates to a current investigation or to leads being checked out.
The new rules allow agents to conduct "general topical research" and "pure surfing" designed to find Web sites, chat rooms or Internet bulletin boards with information about terror, bomb-making instructions, child pornography or stolen credit cards.
The new rules also will make it easier for FBI agents to begin and pursue terrorism investigations without approval from FBI headquarters; give local FBI officials more authority to approve undercover operations in emergency situations; and let agents conduct preliminary investigations for up to six months without special approval from headquarters.
Mueller, who took over as FBI director just days before Sept. 11, is the first senior official in the Bush administration to say that counterterrorism investigators might have detected and averted the attacks if they had recognized what they were collecting. That question is the focus of a congressional inquiry, and almost certain to come up next week during Senate Judiciary Committee hearings on the FBI's reorganization plans.
Mr. Bush has bristled over suggestions that the government had collected enough clues to stop the attacks. "Had I known that the enemy was going to use airplanes to kill on that fateful morning, I would have done everything in my power to protect the American people," Mr. Bush said earlier this month.
The FBI disclosed two other clues Wednesday that it said might be relevant to the investigation into the September hijackings. A Middle Eastern country where U.S. shipments are restricted sought unsuccessfully before Sept. 11 to buy a commercial flight simulator, and an FBI pilot in 1998 expressed concerns to a supervisor in Oklahoma City about a number of Arab men seeking flight training.
The unidentified pilot told his supervisor "that he has observed large numbers of Middle Eastern males receiving flight training at Oklahoma airports in recent months," according to a copy of the one-page memo, under the heading "Weapons of Mass Destruction." The FBI memo, dated May 18, 1998, was marked "routine" and never was forwarded to headquarters.
The pilot added that "this is a recent phenomenon and may be related to planned terrorist activity." He also "speculates that light planes would be an ideal means of spreading chemical or biological agents."
The FBI would not identify the country that sought to buy the simulator except to say it was not one publicly connected to Sept. 11. It said the information was given to the FBI by another U.S. agency that it would not identify.