Special Counsel Robert Mueller’s team has interviewed Deputy Attorney General Rod Rosenstein regarding Donald Trump’s decision to fire former FBI Director James Comey, the Wall Street Journal reports.

Rosenstein was tapped to oversee the Department of Justice’s investigation into Russian meddling and possible collusion with the Trump campaign after Attorney General Jeff Sessions recused himself from the probe. Rosenstein is also the person who appointed Mueller to lead the special counsel after he penned a memo that detailed Comey’s failures in handling the FBI’s investigation into Hillary Clinton’s private email server. That memo was then cited by Trump as a essential to his decision to fire Comey.

Two days after referencing Rosenstein’s memo in his letter ousting Comey, Trump admitted he was considering the Russia probe when he opted to let the FBI director go.

“I said to myself, I said, ‘You know, this Russia thing with Trump and Russia is a made-up story, it’s an excuse by the Democrats for having lost an election that they should have won,’” Trump told NBC News.

Mueller is investigating the circumstances surrounding that decision. According to the Journal, the special counsel likely does not view Rosenstein as a key witness in the investigation, as the deputy attorney general has not recused himself from that investigation, and Mueller still reports to him.

The interview occurred in June or July, sources told the Journal.

“It is unusual,” former federal prosecutor Peter Zeidenberg told the Journal. “But my inference is that they are not viewing him as a potential critical witness, because either the testimony isn’t that critical or there are other people that can say the same thing.”