GOP Sen. Lindsey Graham (S.C.) is questioning if Deputy Attorney General Rod Rosenstein can remain in charge of special counsel Robert Mueller’s probe into the 2016 election.

Graham said he believes Rosenstein is “conflicted” because of the Justice Department official’s role in the firing of then-FBI Director James Comey. Rosenstein drafted a memo that the Trump administration initially used to justify the firing.

“If you’re looking at obstruction of justice misconduct post-presidency, the Comey firing as being a form of obstruction of justice, then Rosenstein is a key witness in that and you can’t be a witness and oversee the investigation,” Graham told reporters on Tuesday.