U.S. Attorney Thomas O’Brien has taken on international drug cartels, some of the West Coast’s toughest street gangs, and human smuggling rings. But now he has to do something that may be even more challenging: build a book of business.

Paul, Hastings, Janofsky & Walker announced Tuesday that O’Brien, who has been U.S. Attorney for the Central District of California since 2007, will become a partner in the firm’s Los Angeles office, where he will focus on white-collar cases.