WASHINGTON – FBI director Robert Mueller said Wednesday that the best advice he can give his successor is to "be flexible and agile" when responding to threats of terrorism.

Mueller, who took the helm of the US Federal Bureau of Investigations one week before the September 11, 2001 attacks, is scheduled to leave his job after mid-year.

It's important for the FBI "to understand what is necessary to protect the American public, to grow and adjust to the new threats that are coming so much faster than they did 10, 15 or 20 years ago, and be flexible and agile to address those threats," Mueller said at a hearing of the Senate Judiciary Committee.

Senators questioned Mueller about three Patriot Act measures that are scheduled to expire in May. The senators are debating whether to temporarily or permanently renew the controversial measures.

The Patriot Act, which Congress approved in the days after 2001 terror attacks, gives law enforcement agencies broad authority to search phone and e-mail messages, as well as personal financial and medical records in their search for terrorist suspects.

In response to a question by Republican Senator Charles Grassley, Mueller agreed that the business records, roving wiretap and "lone wolf" provisions of the Patriot Act should be permanently renewed.

It is "absolutely essential" that US agents have the ability to gather records through the business records provision, Mueller told the senators. "These records enable us to get hotel records, travel records and the like."

Without that capacity "it would be difficult to develop the cases and the investigations in that arena, as well as the counterterrorism arena," he said.

Mueller said his agency has used the roving wiretap provision more than 190 times.

Agents must first show a court that the person they want to wiretap is trying to avoid surveillance. "It has been very helpful on national security and important," he said.

The "lone wolf" provision has yet to be used, Mueller said. "But I still believe that it is important. We have come close to using it in several of our cases."

The measure allows authorities to investigate a person with no known links to a broader organization who is suspected of preparing to carry out a terror attack.

The measure will only be used on a non-US citizen and with prior court approval, Mueller said.

"While we have not used that provision, with the profusion of lone wolf cases domestically and, indeed, some internationally, my expectation is we will be using this in the future," he said.

Critics believe that after ten years it is time to let the measures expire.

Mueller said he believes there has been an "appropriate balance" between "the necessity for addressing the terrorist threat and threat from other criminal elements to the United States, and ... the protection of privacy, civil liberties."

Those being considered for his replacement, according to US news media, include former FBI deputy director John Pistole, and Patrick Fitzgerald, the US attorney famous for investigating the Valerie Plame affair, which led to the conviction of then-vice president Richard Cheney's chief of staff Scooter Libby.