Deputy Attorney General Rod Rosenstein Rod RosensteinDOJ kept investigators from completing probe of Trump ties to Russia: report Five takeaways from final Senate Intel Russia report FBI officials hid copies of Russia probe documents fearing Trump interference: book MORE tapped a longtime prosecutor to assist him in overseeing the major federal probes currently underway.

Rosenstein chose Ed O'Callaghan to fill the role of acting principal associate deputy attorney general.

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"His experiences in a variety of roles throughout the Department will be invaluable as we work to protect our national security, reduce violent crime, and promote the rule of law,” Rosenstein said in a statement.

O'Callaghan's new role will involve him advising Rosenstein on all major investigations, which will include the federal probe into Russia's election meddling led by special counsel Robert Mueller Robert (Bob) MuellerCNN's Toobin warns McCabe is in 'perilous condition' with emboldened Trump CNN anchor rips Trump over Stone while evoking Clinton-Lynch tarmac meeting The Hill's 12:30 Report: New Hampshire fallout MORE.

He replaces Robert Hur, who confirmed as the U.S. attorney for the district of Maryland in March.

O'Callaghan most recently served as the acting head of the department's national security division.

The appointment comes a day after Rosenstein gave Mueller the authority to investigate alleged collusion between former Trump campaign chairman Paul Manafort Paul John ManafortOur Constitution is under attack by Attorney General William Barr Bannon trial date set in alleged border wall scam Conspicuous by their absence from the Republican Convention MORE and the Kremlin.

Manafort was indicted last year as a part of Mueller's probe on money laundering charges and other alleged financial crimes.

Mueller is in the process of probing Russia's election interference and possible ties between the Trump campaign and Moscow.

Trump has denied collusion between his presidential campaign and the Kremlin and has referred to Mueller's probe as a "hoax" and a "witchhunt."

Updated: 6:50 p.m.