U.S. Deputy Attorney General Rod Rosenstein has privately acknowledged he may need to recuse himself from matters relating to the Russia probe, given that he could become a potential witness in the investigation, ABC News reported on Friday, citing unnamed sources.

ABC said Rosenstein told Associate Attorney General Rachel Brand she would have authority over the probe if he were to step aside. Rosenstein appointed special counsel Robert Mueller to investigate alleged Russian meddling in the 2016 U.S. election, and has told lawmakers he would only fire him with good cause.

The report came after U.S. President Donald Trump acknowledged for the first time on Friday that he was under investigation over the firing of FBI Director James Comey, while also appearing to attack Deputy Attorney General Rod Rosenstein.

“I am being investigated for firing the FBI Director by the man who told me to fire the FBI Director! Witch Hunt,” Trump tweeted.

It was a memo written by Rosenstein that was originally used by the White House as justification for Comey's firing. It was not clear whether Trump's tweet was referring to Rosenstein or Mueller, but most news organizations identified the former as Trump's target.