The U.S. Attorney's Office for the District of Columbia is losing another top prosecutor to a California-based law firm that is building up its Washington practice.

Daniel Seikaly, who headed the office's criminal division, is the third such hire by the firm of Sheppard, Mullin, Richter & Hampton, which opened a full-service Washington office last year. He starts work Aug. 3, the firm said.

Former U.S. attorney Roscoe C. Howard Jr., who had been the District's top prosecutor, joined the firm last month, along with Mark E. Nagle, who headed the civil division under Howard.

The hirings represent an aggressive strategy by Sheppard Mullin to assert itself as a leading Washington firm. The firm has more than 400 lawyers in eight offices, and the Washington office now has more than 25 lawyers who practice a full range of specialties.

"Dan's vast experience -- including positions dealing with criminal, fraud and national security issues -- further heightens Sheppard Mullin's profile in the D.C. legal community and strengthens the firm's criminal defense practice," said Edward F. Schiff, head of the firm's Washington office.

Seikaly, 58, has headed the criminal division at Washington's U.S. Attorney's Office since 2001. He supervised more than 80 prosecutors.

He came to the U.S. Attorney's Office in 1980, handling an array of criminal trials, and became chief of the D.C. office's transnational and major crimes division in 1989. In that assignment, he handled numerous high-profile cases, including investigations of the 1988 downing of Pan American Flight 103 over Lockerbie, Scotland, and of the 1985 hijacking of TWA Flight 847. U.S. Navy diver Robert Dean Stethem was killed in that hijacking.

In 1996 Seikaly moved to the Justice Department, where he worked as associate deputy attorney general and director of the executive office for national security. In 1998 he went to the CIA, where he was assistant inspector general for investigations until 2001.

"I was attracted to Sheppard Mullin's unique vision, and the fact that I get to work with two of my longtime colleagues is an added bonus," Seikaly said. "Sheppard Mullin has committed itself to developing a top-notch firm in D.C., and getting into that office early was an attraction."

Channing D. Phillips, spokesman for the U.S. Attorney's Office, said the departures would not hurt the agency's operations.

"Our office is accustomed to law firms and other government agencies hiring many of our top litigators, simply because when you practice in the District of Columbia, you are practicing before one of the top courts, as well as going up against some of the top attorneys in the country.

"Given the size of our office -- 350-plus assistant U.S. attorneys -- and the talented pool that we have, we certainly don't think we'll miss a beat in serving the residents of the District of Columbia."