Edward O’Callaghan, a top U.S. Justice Department official in the Trump administration who played a lead role overseeing the Russia investigation, has joined the law firm Wilmer Cutler Pickering Hale and Dorr, a move that returns him to working alongside former Special Counsel Robert Mueller III.

Before stepping down in December, O’Callaghan served as the chief liaison between Mueller’s team and the Justice Department leadership in his role as the top aide to then-Deputy Attorney General Rod Rosenstein. O’Callaghan, a former federal prosecutor in New York, initially headed the national security division when he returned to the Justice Department as a political appointee during the first year of the Trump presidency.