BOSTON (AP) — A federal prosecutor who recently completed a 20-year quest to convict former Boston crime boss James "Whitey" Bulger has left the U.S. Attorney's Office to join a prominent Boston law firm.

Brian Kelly will join Nixon Peabody LLP next month as a partner in the firm's Government Investigations and White Collar Defense practice.

Kelly, an assistant U.S. Attorney, was one of three federal prosecutors who won a racketeering conviction against Bulger in August. Bulger, one of the nation's most wanted fugitives for more than 16 years, was sentenced last month to life in prison.

Kelly has served as lead counsel in more than 30 federal trials. He supervised the investigation and prosecution of political corruption cases, including the convictions of Massachusetts House Speaker Salvatore DeMasi and House Speaker Tom Finneran.