ASHINGTON, June 1 — When the director of the F.B.I., Robert S. Mueller III, acknowledged on Wednesday that the agency had missed warning signals on terrorism, he stunned many Americans. But his statement was not news to some veterans of the agency — or lawmakers who now say they treated the F.B.I. with too much deference for too many years.

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These officials say the F.B.I., despite efforts to strengthen its counterterrorism programs over the last decade, and despite hefty increases in its budget, never developed a nimble enough structure, analytic capability or sense of mission to foil terrorist plots before they were carried out.

Interviews with nearly two dozen current and former F.B.I., Justice Department and intelligence officials, many of them at a senior level, suggest that Mr. Mueller faces many hurdles in fulfilling his promise to transform the agency's rigid, risk-averse culture into the kind of terror prevention agency he foresees. Some officials even question whether the bureau can be salvaged, or whether it should be broken apart so that the government can create a domestic intelligence agency separate from the F.B.I.

"There's got to be follow-through on this reorganization," said Robert S. Bryant, a former deputy F.B.I. director. "This isn't a law enforcement issue. We are at war. We've got to get more information. There has to be discipline to stay at it and pull it all together from the F.B.I. and other sources in the government."

In announcing the reorganization this past week, Mr. Mueller said the bureau would hire 400 more analysts, including 25 officers to be borrowed from the Central Intelligence Agency. He also announced plans to establish "flying squads" of terrorism experts based at F.B.I. headquarters, who would feed intelligence to field offices.

In an interview today, Mr. Mueller said the changes would vastly enhance the F.B.I.'s ability to thwart terrorists. He offered no assurance that the bureau would ever eliminate all terrorist threats.

Still, he said his reorganization proposal — unlike past restructuring efforts — had a better chance of success because the hijackings drove home the realization that change cannot wait.

"It's a combination of circumstances, but I'm certain that Sept. 11 has had a dramatic effect on every member of the F.B.I. to do everything we can to prevent any additional terrorist attacks," he said. "So there is an openness and willingness to change and a new understanding of the threats we face in the future."

Agents realize, Mr. Mueller said, that the old criteria for success within the F.B.I. no longer apply.

"We've come to understand that we are not going to be judged in the future by how many successful prosecutions we have of terrorists, but will be judged by our capacity to prevent additional terrorist attacks," he said. "It's picking up information that may assist in preventing terrorist attacks and moving it to where it can help."

Hiring hundreds more agents, analysts and linguists may be the easiest fix. Far more difficult, many officials say, is the challenge of remaking the F.B.I.'s dysfunctional bureaucracy.

"Twenty-five years ago, the thought was you had to tame down the F.B.I., they were out of control," one retired senior F.B.I. official said.

"But in the last 15 years, we have become a very docile, don't-take-any-risks agency, particularly at headquarters. And if you make a mistake and it blows up in your face, then your career is shot, because basically it's one strike and you are out of the F.B.I. All that has to change."

One indicator of the paralytic fear of risk-taking was how F.B.I. headquarters responded to the memorandum written last July by an agent from Phoenix. The agent, Kenneth J. Williams, urged a broad survey of American aviation schools based on his concern that Middle Eastern men, possibly connected to Osama bin Laden, were training at a flight school in Arizona.

But officials at headquarters rejected his proposal. Mr. Mueller has said that the plan was deferred for lack of resources. But other officials pointed to another reason: the worry that such an effort might be criticized in Congress as racial profiling. Mr. Williams's idea died, until after the Sept. 11 terrorist attacks.

The problems have been apparent for years. In 1999, the chief of the F.B.I.'s counterterrorism unit, Dale Watson, concluded that too few agents around the country were working to thwart terrorism. In March 2000, he convened a meeting at headquarters of the agents in charge of all 56 field offices. Some agents called the meeting "Terrorism 101" or "Terrorism for Dummies."

Mr. Watson and other senior officials were startled to learn how little some bureau offices around the country, operating independently of headquarters, had done to investigate terrorism.

Even after the meeting, in the months before Sept. 11, senior agents at headquarters were reduced to repeatedly cajoling the special agents in charge of the field offices to work harder on counterterrorism inquiries. They even threatened to withhold managers' raises and bonuses if they did not pay more attention to the problem.

Beyond the issue of whether the agency can fix itself is a political question.

Will Mr. Mueller, a former United States attorney, and his top deputies maintain the support of the Bush administration, particularly in the face of skeptical Congressional inquiries into what they knew and when they knew it in the weeks and months before Sept. 11?

The F.B.I.'s current state — so unready, so unprepared and so unable to assess the accumulating warning signs of the hijackings — is the result of years of neglect by the successors to J. Edgar Hoover, who ran the agency for 48 years. Each director missed repeated opportunities to change a law enforcement agency that many critics believe was better suited to catching criminals of the Bonnie and Clyde era than trying to prevent crimes plotted by Osama bin Laden's terrorism network.