Alan Dershowitz said fired FBI agent Peter Strzok made one serious mistake that led to his termination: failing to recuse himself in the Russian meddling and Hillary Clinton email probes.

Dershowitz explained that Strzok -- who infamously exchanged anti-Trump text messages while he was playing a lead role in the Russia and Clinton investigations -- created the appearance of bias.

According to a bombshell report by the Justice Department's inspector general, bureau attorney Lisa Page texted Strzok in August 2016 to ask: “[Trump’s] not ever going to become president, right? Right?!”

“No. No he won’t. We’ll stop it,” responded Strzok, who was having an extramarital affair at the time with Page.

Dershowitz, a Harvard Law professor emeritus, said it's unknown if Strzok's apparent anti-Trump bias specifically motivated any of his official actions, but he needed to remove himself from the investigations to preserve the FBI's appearance of impartiality.

Giuliani: John Brennan Led the Effort to Start Trump-Russia Investigation

Son of GOP Rep. Goodlatte Calls Peter Strzok a 'Patriot,' Blasts His Father

"He should have recused himself. He's a professional. And his failure to recuse himself, I think, is what led to his firing," Dershowitz said.

He added that this raises an important question: why hasn't Deputy Attorney General Rod Rosenstein recused himself from Special Counsel Robert Mueller's Russia investigation?

"He also creates the impression of bias. Not, again, from personal feelings but because he's a witness in the case, he's the main witness in the case," Dershowitz said. "You can't be the main prosecutor and the main witness in the same case. That creates an obvious conflict of interest."

See Dershowitz's full analysis above.

'Architect of the Collusion Theory': Fitton Says Strzok's Firing Is 'Body Blow' to Mueller Probe

'Why Did It Take This Long?': 'Outnumbered' Sounds Off on Peter Strzok Firing

Tucker Highlights Antifa Protesters Threatening to Murder Trump, Chanting 'No USA At All!'

Tomi Lahren: 'Unite the Right' and Antifa 'Disgusting', But Media 'Gives One a Pass'