(CNN) As Robert Mueller's probe reaches those in President Donald Trump's inner circle, former US attorney Preet Bharara says he is concerned that the President could interfere in special counsel Robert Mueller's investigation.

"I would worry in a real way that Donald Trump may preemptively pardon some people, and I still worry in a real way that Donald Trump may decide to cause the firing of Robert Mueller," Bharara told David Axelrod on "The Axe Files," a podcast from the University of Chicago Institute of Politics and CNN.

Bharara, a former US attorney in New York who himself was ousted by Trump , said the President has already demonstrated his willingness to exercise his "full constitutional authority" to both pardon and fire people. Bharara noted that fraught political situations, as with the pardon of Sheriff Joe Arpaio and the firing of FBI Director James Comey , do not seem to dissuade Trump from exercising those political powers.

Reacting to former national security adviser Michael Flynn's guilty plea, which occurred just hours before the "Axe Files" interview, Bharara said there are still a number of questions to be answered. Flynn pleaded guilty on Friday to lying to the FBI about conversations with Russian Ambassador Sergey Kislyak. He is the first person who held a position inside Trump's administration to be reached by Mueller's probe.

One of the biggest of those questions, Bharara suggested, was why did Flynn lie to the FBI?

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