U.S. Attorney Roscoe C. Howard Jr. said last night that he is stepping down after nearly three years as the chief prosecutor for the District to become a partner in the Washington office of a California law firm.

With Howard's departure next week, one of the most important federal prosecutor jobs in the country will be open. Selected by President Bush, he oversaw the prosecution of local and federal crimes in the nation's largest U.S. attorney's office, handling matters ranging from the anthrax investigation to the prosecution of local gangs.

No replacement has been named, and the Bush administration will choose an interim leader to head the office. With the presidential election less than six months away, it is possible that a permanent successor will not take office until after the 2005 inauguration.

Howard, 52, said he is joining the firm of Sheppard, Mullin, Richter & Hampton, effective June 1. The firm has more than 400 attorneys in eight offices, with a broad mix of specialties, including antitrust, government contracts and mergers and acquisitions. Howard will be taking with him one of his top deputies, Mark E. Nagle, the chief of the civil division. The firm opened its Washington office last year and now has about 20 lawyers there.

Guy N. Halgren, chairman of Sheppard Mullin's executive committee, said last night that hiring Howard was a "coup," adding: "No question. There's only one U.S. attorney in every city, and Roscoe was the U.S. attorney for Washington, D.C."

Confirmed by the Senate just a few days after the Sept. 11, 2001, terrorist attacks, Howard oversaw an expansion of the role of the U.S. attorney's office in the government's counter-terrorism efforts -- a role that has brought with it numerous challenges and high-profile cases.

Howard's office has played a central role in the investigation of the anthrax mailings that killed five people and sickened 17 others in the fall of 2001. Although the case has generated no arrests, it has nonetheless consumed time in court.

Former Army scientist Steven J. Hatfill, once described by Attorney General John D. Ashcroft as a "person of interest" in the case, is suing the Justice Department for defamation.

The pace of the investigation has left many unanswered questions for a community that was stricken with fear by the anthrax attacks. In an interview last night, Howard said the investigation was moving forward. He said he wished it had progressed further.

"We have a great team of agents from the FBI, the Postal Service and prosecutors from our office, and they're doing a great job," he said. "It's just a tough, tough case. We're really working really hard to come to some resolution."

On a local level, Howard's job became more challenging because of the Sept. 11 attacks. The FBI, once a key partner with the D.C. police in investigating complex drug, corruption and organized crime cases, pulled back considerably as it focused on terrorism. The FBI and other agencies have since shifted some attention back to local matters, but they remain hard-pressed because of continuing terror concerns.

Under Howard's leadership, prosecutors recorded some significant victories in local cases. They won convictions after two marathon trials of members of a drug gang known as Murder Inc., which authorities said was responsible for 31 killings.

And last year, they obtained a guilty plea from Barbara A. Bullock, the former president of the Washington Teachers' Union, who admitted stealing millions of dollars in dues to pay for furs, jewelry and other extravagances.

Criticized in some quarters for moving too slowly to build public corruption cases, Howard said that he would have liked to have seen more such prosecutions but that the changed law enforcement landscape made that more challenging.

"Clearly there could have been more white-collar cases," he said. "The investigators that we normally would turn to for our white-collar investigations weren't there."

Howard, an assistant U.S. attorney in the District and in Virginia for several years, left government in the mid-1990s to join the University of Kansas School of Law, where he was teaching when Bush tapped him to head the office with about 350 lawyers.

He also has spent some time in private practice.

Some community activists have pushed for the local prosecutor to be locally selected -- either by the people or by the mayor, as is the case nearly everywhere else in the country. Voters in the District passed a measure in 2002 expressing support for the idea of a locally elected prosecutor.

Roscoe C. Howard Jr. said he is joining the D.C. office of a California firm.