In the last two weeks, outside prosecutors have been questioning prosecutors in Washington about different cases, seeking information regarding investigative procedures and prosecutorial decisions, the report said citing people familiar with the matter.
The report said that one of the prosecutors assigned to review the Flynn case is from the US Attorney's office in St. Louis, Missouri.
Barr has also assigned other outside prosecutors to review other politically-sensitive national security cases, the report said. Several prosecutors are from the office of US Deputy Attorney General Jeffrey Rosen, the report added.
President Donald Trump has recently voiced his objection to the sentencing recommendation of his former campaign adviser Roger Stone. His statements were followed by Barr's decision to step in on the case to lighten his sentencing, which spurred the resignation of four prosecutors on the case.
Flynn pleaded guilty to lying to the FBI about holding conversations with then Russian Ambassador to the United States Sergey Kislyak, including regarding the prospects for lifting US sanctions against Russia.
Flynn originally agreed to cooperate with the investigation of Special Counsel Robert Mueller into allegations of collusion between the 2016 Trump campaign and Russia. The investigation found no evidence of collusion.
Russian officials have repeatedly said Russia does not interfere in the US political system and the allegations of collusion were made up to excuse the election loss of Trump’s opponent Hillary Clinton, as well as deflect public attention from instances of election fraud and corruption.