Thomas P. O'Brien, the Los Angeles-based U.S. attorney for the Central District, will be stepping down to practice law at a private firm, according to officials.

O'Brien will leave Sept. 1 to take a job with the Paul Hastings law firm, said Thom Mrozek, spokesman for the U.S. attorney's office in Los Angeles.

A former Navy Top Gun instructor and gang prosecutor in the district attorney's office, O'Brien led an office that took on a number of high-profile prosecutions.

Sworn in as a U.S. attorney in October 2007, O'Brien oversaw the grand jury investigation of Cardinal Roger M. Mahony's handling of molestation by priests in the Archdiocese of Los Angeles. Also, federal prosecutors in Los Angeles aggressively pursued local street gangs during his tenure, including a sweeping racketeering case against a Hawaiian Gardens gang that O'Brien described as the "largest gang take-down in U.S. history."

O'Brien graduated from the United States Naval Academy in 1981 and received his law degree in 1991 from the University of San Diego School of Law. He was on a flight Monday evening and unavailable for comment.

-- Robert J. Lopez

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