Senate Majority Leader Mitch McConnell (R-Ky.) said Thursday that he doesn’t believe President Trump will stop special counsel Robert S. Mueller III’s Russia investigation, one day after Trump’s ouster of Jeff Sessions as attorney general threw the future of the probe into uncertainty.

Trump appointed Matthew G. Whitaker, Sessions’s chief of staff, on Wednesday to serve as acting attorney general. Whitaker has faced questions about whether he should recuse himself from the investigation, given that he has written opinion pieces about the probe and is a friend of a witness, but people close to him say he has no intention of doing so.

“The president has said on multiple occasions the Mueller investigation should be completed,” McConnell said in an interview with Lexington, Ky.-based radio station WVLK, according to CBS News. “He [wishes] it would happen sooner. But I don’t think there’s any chance that the Mueller investigation will not be allowed to finish.”

Trump has repeatedly denounced the Mueller investigation into Russian interference in the 2016 election as a “witch hunt” and has complained that it should have wrapped up by now. The president has also had a combative relationship with Deputy Attorney General Rod J. Rosenstein, who had overseen the probe since Sessions’s recusal last year. Whitaker is now expected to assume that role.

Democrats and some Republicans have voiced concern about the future of the investigation, warning on Wednesday that the Trump administration must allow Mueller to complete his work.

But questions continued to swirl Thursday around Whitaker’s role in the probe. Two people close to Whitaker told The Washington Post they strongly believe he would not approve any request from Mueller to subpoena the president.

Some critics have also argued that Trump’s installation of Whitaker is unconstitutional.

“It’s illegal. And it means that anything Mr. Whitaker does, or tries to do, in that position is invalid,” former acting solicitor general Neal K. Katyal and George T. Conway III, husband of White House counselor Kellyanne Conway, wrote Thursday in a New York Times op-ed.

Devlin Barrett, Matt Zapotosky, Josh Dawsey and John Wagner contributed to this report.