Frank W. Dunham Jr., who created the federal public defender's office in Alexandria and represented some of the nation's best-known terrorism defendants, is retiring because of a long-term illness, officials said yesterday.

Dunham notified the U.S. Court of Appeals for the 4th Circuit on Monday that his retirement as federal public defender will take effect Feb. 1, Chief Judge William W. Wilkins Jr. said yesterday. Wilkins said Dunham, 63, cited the illness and his desire to spend more time with his family.

The retirement caps a high-profile career for Dunham, who started the public defender's office four years ago in a small room at the Alexandria federal courthouse. From that beginning, he built a practice that represents indigent defendants and now handles about 60 percent of the cases at the busy courthouse, which serves all of Northern Virginia.

Dunham's office has achieved national recognition for handling a raft of cases in what has become the Justice Department's venue of choice for major terrorism and national security prosecutions. His clients have included Sept. 11 conspirator Zacarias Moussaoui and "enemy combatant" Yaser Esam Hamdi.

In Moussaoui's case, Dunham and his team of attorneys battled the government to a virtual standstill for two years before Moussaoui pleaded guilty in April. Dunham also personally argued before the U.S. Supreme Court the case of Hamdi, a U.S. citizen held as a combatant by the military. That produced an important decision that upheld the government's power to detain Hamdi but said he could challenge that detention in U.S. courts.

As the legal war on terrorism escalated, "Frank seized the moment that was before him,'' said Paul J. McNulty, the U.S. attorney in Alexandria. "He found ways to really put himself out there, sometimes to the frustration of my prosecutors, but always in a way that balanced the seriousness of the issue with his concerns for proper procedure.''

McNulty added that Dunham, a former federal prosecutor and Arlington-based defense lawyer, "combines a heart for the accused with a genuine appreciation of law enforcement.''

Dunham could not be reached for comment yesterday.

The Richmond-based 4th Circuit appoints the federal public defender, and Wilkins said the process of selecting a replacement has already begun. Wilkins said he will appoint a committee made up of a variety of lawyers and will advertise the position in legal and other publications. The committee will select whom to interview, and the full slate of 4th Circuit judges will make the final decision.

"Frank has done an excellent job in that office. He really represents his clients very well,'' said Wilkins, who plans to appoint an acting public defender when Dunham retires.

Michael Nachmanoff, the first assistant federal public defender, has been running the office since Dunham fell ill in May and said yesterday that he will continue to do so. "The chance to work with Frank has been unrivaled,'' Nachmanoff said. "He's a terrific lawyer and a terrific leader.''

Dunham's successor will inherit an office whose 24 attorneys are vastly outnumbered by the Alexandria-based U.S. attorney's office, which has increased its staff more than 20 percent since 2001. Caseloads are also exploding. Federal officials originally predicted that the public defender's office -- which also has divisions in Richmond and Norfolk -- would handle about 1,500 cases a year.

The actual number this year, Nachmanoff said, will be about 2,200 cases.

The Eastern District of Virginia was one of the last large judicial districts in the country to open a defender's office. When Dunham was sworn in on March 15, 2001, he recalled in an interview last year, "it was me, myself and I." Even the old wooden filing cabinet in his courthouse office didn't work.

Dunham said he grew to love the work, calling it some of "the most exciting years I've had practicing law.''

Frank W. Dunham Jr. "combines a heart for the accused with a genuine appreciation of law enforcement," said U.S. Attorney Paul J. McNulty.