The director of the FBI, Robert Mueller, has appeared before a congressional panel to explain plans to reorganize the agency. Lawmakers want to know what changes will be carried out in the much-criticized intelligence agency, and also expressed concern the FBI efforts to fight terrorism don't come at the expense of civil liberties.

Mr. Mueller began by observing that preventing terrorist attacks will be a way of life for America's intelligence agencies far into the future. But doing it right, he added, is just as important.

That was a reference to concerns that the ambitious restructuring of the FBI, including loosened restrictions on intelligence gathering, may result in violations of civil liberties.

"There will always be tension in a free and open society between national security and personal liberties," New York Democratic Congressman Jose Serrano said. "My fear now is that the delicate balance is shifting."

Mr. Mueller says he believes there are legal and other constraints already in place to prevent the FBI from overstepping its bounds. But he says the job will not be easy.

"We have to be watchful, we have to monitor what our agents are doing. It is particularly difficult when you are looking at terrorism and trying to be predictive and preventive, because there is no one as yet who has committed a crime," he said. "There is a very fine line we walk in terms of surveilling an individual who might commit a terrorist act and surveilling somebody who is a private citizen who is acting a little bit differently."

Mr. Mueller says reducing bureaucracy and increasing accountability are key goals of the reorganization announced several weeks ago.

A joint House-Senate congressional committee has been investigating how the FBI, as well as CIA, mishandled information prior to the September 11 terrorist attacks in New York and Washington.

One of the most controversial incidents involved a memo from the agency's Phoenix, Arizona, field office, which was not given a high priority at upper levels of the FBI.

Referring to this and other problems, Mr. Mueller described what he said is perhaps the most urgent goal of the reorganization.

"Building a robust, integrated, analytical capacity that not only will support our own investigative and preventing needs but also will complement and enhance the efforts of the CIA as well as those envisioned in the new homeland security agency," he said.

Just after the September 11 attacks, Mr. Mueller said 6,000 FBI agents were shifted to counter-terrorism. That number is now down to 2,000.

Several lawmakers expressed concern that other aspects of the FBI's work, such as counter-narcotics, will not suffer in the reorganization. Mr. Mueller says he does not want to move "too far, too fast" in permanently reassigning agents.

Critics say a new cabinet-level homeland security department can't be effective if it cannot have access to all information flowing into the FBI, CIA, and other intelligence agencies.

Mr. Mueller told lawmakers that would be "impractical," a response that did not appear to satisfy congressional questioners.