Washington (CNN) Deputy Attorney General Rod Rosenstein has picked a veteran prosecutor to help him oversee the Russia probe at the Justice Department as the Special Counsel's investigation deepens.

Ed O'Callaghan will serve as the acting Principal Associate Deputy Attorney General, according to a Justice Department official.

The powerful post, commonly known as the "Padag" within the department, advises the Deputy Attorney General on all major investigations and policy matters, but generally floats under the radar.

Yet O'Callaghan now lands in the thick of the highest-profile investigation at the Justice Department, ensnarling former Trump campaign associates. Late Monday evening, prosecutors in the Russia probe revealed Rosenstein has granted the special counsel's office wide latitude to investigate former Trump campaign chairman Paul Manafort's connections to Russian government officials. Thus far, the special counsel's office has charged 19 people and three companies, and one person has been sentenced

A long-time federal prosecutor and counterterrorism expert in New York, O'Callaghan most recently served as the acting head of the DOJ's National Security Division. He raised eyebrows of some former DOJ officials earlier this year when he took to the White House podium to defend the Trump administration's use of contested terrorism figures to encourage changes to immigration policies.

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