ASHINGTON, Oct. 20 ?The Bush administration is discussing proposals that would lead to the most fundamental reorganization of the Federal Bureau of Investigation in its history, shifting its focus to counterterrorism and away from crime fighting, senior officials said.

Under the new thinking, they said, the agency would give up responsibility for some of the duties on which it built its legendary "G-man" reputation, like bank robbery, drug trafficking and some violent crime investigations.

"As counterterrorism becomes the No. 1 priority of the F.B.I., it has become obvious that other types of investigations will have to be de-emphasized at the bureau or turned over to other agencies," said a senior administration official, one of several interviewed in recent days who have been involved in the discussions.

Some officials say the restructuring has already begun, even before any formal plans have been proposed, propelled by the terrorist attacks of Sept. 11, when thousands of bureau agents across the country were ordered to put aside other investigations to focus exclusively on counterterrorism.

Since Sept. 11, senior officials said, Attorney General John Ashcroft and the bureau's director, Robert S. Mueller III, have agreed that the emphasis on counterterrorism will be permanent, and that other major changes are inevitable. They have said repeatedly in recent days that the bureau's 28,000 employees will have one overriding responsibility: to prevent further terrorist attacks against Americans.

The Associated Press Attorney General John Ashcroft, left, with the F.B.I.'s director, Robert S. Mueller III, last week.

Officials emphasized that no formal restructuring plan exists, and that any structural change in the bureau's mission might require Congressional approval.

But the trauma of Sept. 11 appears to give this proposal a far better chance of success than many of the other ideas that repeatedly arise in Washington to remake complicated or failing bureaucracies, like the perennial plans to restructure the Immigration and Naturalization Service.

The attorney general and bureau director strongly support the change, law enforcement officials said. And because of the investigation of the terrorist attacks, some of the ideas are already being put into place ?a de facto restructuring.

In addition, even before Sept. 11, members of Congress in both parties were calling for significant change at the bureau. Since the attacks, they have praised Mr. Ashcroft for his insistence that the bureau concentrate on preventing terrorist acts.

"That's exactly what he ought to be doing," said Senator Jon Kyl, a Republican of Arizona who is a member of both the Intelligence and Judiciary Committees. "What's important now is to track down and prevent more terrorism."

For generations, career advancement at the bureau has depended on the sort of basic gumshoe investigations that would now be turned over to other federal agencies or even to local police departments. For that reason, the change is already facing opposition from the rank and file at the bureau, one of the government's most tradition-bound agencies.

Until now, agents who worked in the "other side" of the bureau ?in the classified world of counterterrorism and counterintelligence units ?seldom gained the same renown or promotions as their counterparts in the criminal division. Frequently, the bureau's counterintelligence agents complained that their biggest successes were necessarily cloaked in secrecy.

Their problems were compounded by management changes in recent years that granted flexibility to supervisors in the bureau's 56 national field offices to set their own priorities, a system that in some places resulted in a downgrading of counterterrorism and counterintelligence investigations.

But in the aftermath of Sept. 11, senior administration officials say, counterterrorism and counterintelligence must be the bureau's principal responsibilities.

As a result, they added, the bureau will need to give up responsibility ?permanently ?for many types of more routine criminal investigations. The bureau has already directed agents to stop responding to nonviolent bank robberies, so-called note jobs.

Administration officials said that under a reorganization, many types of narcotics investigations that had previously been handled by the bureau would very likely be turned over to the Drug Enforcement Administration, and that the Bureau of Alcohol, Tobacco and Firearms would take over some firearms and bombing cases previously handled by the bureau.

The F.B.I., they said, may jettison some of the jurisdiction that J. Edgar Hoover and his successors had won for the bureau, which is known in Washington for its aggressiveness in trying to expand its turf and budget, a strategy that Congress has willingly supported with bigger and bigger budgets and more personnel.

Some recently acquired powers may be among the first to be relinquished, like the bureau's responsibility for investigations of child pornography, carjackings and fathers who have not paid child support. In the future, the agency could also give up investigations of health care and military-procurement fraud, duties that could be handed over to the offices of inspectors general at the Department of Health and Human Services and the Pentagon. Another senior official, speaking on the condition of anonymity, said that "12 months from now, the F.B.I. is not going to be the organization it was on Sept. 10."

"Its responsibilities and priorities are all going to change," he added. "Any area where there is a duplicative effort with some other part of the government has a strong chance of being broken off from the bureau's responsibility."

No one in the administration is suggesting that the bureau will be sidelined in the government's effort to combat major crime. But law enforcement officials said that narrowing the bureau's focus would make the agency more effective in responding to crimes that it is uniquely qualified to address, like complex white-collar fraud, organized crime and political corruption.

A major restructuring of the bureau has been under discussion since the early days of the Bush administration. The administration inherited an agency battered by criticism in Congress over missteps that seemed rooted in managerial failures.

Supporters of the F.B.I. in Congress complained of mismanagement after the bureau's erratic investigation of Wen Ho Lee, the Los Alamos nuclear weapons scientist; the unmasking of an F.B.I. agent, Robert P. Hanssen, as a Russian spy; and the belated discovery of investigative documents in the Oklahoma City bombing case that forced Mr. Ashcroft to delay the execution of Timothy J. McVeigh.

In a speech to bureau employees last summer, Mr. Ashcroft said the Hanssen case and the handling of the McVeigh documents were "injuring the public trust" and signaled that he would keep a close watch on the agency. He started several internal inquiries and brought in a private consulting firm to conduct a management review of the F.B.I., which is still under way.

He persuaded a skeptical White House to accept his choice, Mr. Mueller, as the bureau's new director. Mr. Mueller had earned a fearsome reputation from previous jobs for shaking up government agencies, notably the United States attorney's office in San Francisco, where he forced out most of the senior managers. Both Mr. Ashcroft and Mr. Mueller, senior aides said, were determined to end decades of hostility and turf battles between the Justice Department and the F.B.I.

Within hours of the attacks on the World Trade Center and the Pentagon, officials said, Mr. Ashcroft and Mr. Mueller had no choice but to restructure the bureau, at least temporarily.

Thousands of agents were told to abandon their more routine criminal investigations and to focus entirely on terrorism, especially on pursuing leads that might prevent another terrorist attack. A senior Justice Department official said that although there had been no bureauwide notice to those agents of their future duties, "many of them won't be going back to their old jobs."