The resignation of Associate Attorney General Rachel Brand, a national security law expert, comes as President Donald Trump and Republican legislators have stepped up attacks on the department over special prosecutor Robert Mueller's investigation of possible Trump campaign links to Russian interference in the 2016 election.
Brand was in position to inherit oversight of the Mueller probe if the department's number two, Deputy Attorney General Rod Rosenstein, is forced out.
The Justice Department did not immediately confirm the report. The New York Times said Brand is leaving for a job in the private sector.
In 2012 president Barack Obama appointed her to the Privacy and Civil Liberties Oversight Board, which reviewed the legality of the surveillance programs of the National Security Agency, especially its data collection on US citizens, which Brand defended.
She rejoined the Justice Department last May, appointed by Trump and working directly below Rosenstein and Attorney General Jeff Sessions, who recused himself from involvement with the Mueller probe due to his own work for the Trump campaign.
Trump, frustrated by the Russian investigation, has reportedly considered firing Rosenstein, the only person empowered to dismiss Mueller.
A recent memo from congressional Republicans took aim at Rosenstein for his role in obtaining wiretap warrants on a member of the Trump campaign who had numerous Russian contacts, which the memo described as an abuse of power.
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