During an interview with NBC, Trump said "regardless of recommendation, I was going to fire Comey".
Trump has repeatedly denied any collusion with Russians in the run-up to the November 2016 election that he won, but the latest media reports suggest that Mueller may be probing whether he, as President, sought to obstruct the investigation itself.
What began as a probe into whether or not the Trump campaign colluded with Russian Federation to influence the 2016 election has turned into one focused on obstruction of justice.
Pompeo and Dan Coats, the director of national intelligence, who has also been grilled by Mueller, are described as "peripheral witnesses" to Trump's firing of former FBI Director James Comey, NBC News reported Wednesday.
So far no evidence of collusion has come to light, but Mueller's probe has led to indictments against former Trump campaign chairman Paul Manafort and another aide, Rick Gates, and guilty pleas by national security adviser Michael Flynn and another former Trump adviser, George Papadopoulos.
The attorney general was accompanied to the interview by Washington attorney Chuck Cooper and was questioned for several hours, USA Today reported, citing Justice Department spokesman Ian Prior.
Mueller's team is expected to be interested in meetings between Sessions and Kislyak during the campaign, as well as the attorney general's involvement in Trump's firing of Comey, an episode central to the question of whether Trump may have committed obstruction of justice.
Trump said his presidential rival Hillary Clinton did not give interview to Federal Bureau of Investigation under oath.
Asked Tuesday afternoon in the Oval Office if he had talked to Sessions about what the attorney general might have said to the investigators, President Trump said he hadn't.
Director of strategic communications Mercedes Schlapp reacts to the attorney general being questioned in the Russian Federation probe.
President Donald Trump is rejecting criticism of his Russian Federation probe counter-attacks.