Former special counsel Robert Mueller on Wednesday said that he believes President Trump could be charged with obstruction of justice after he leaves office.

"Could you charge the president with a crime after he left office?" Rep. Ken Buck (R-Colo.) asked Mueller during the former special counsel's testimony.

"Yes," Mueller replied.

Buck appeared to be taken aback, and asked the question again, but adding whether Mueller believed a president could be charged with obstruction of justice after leaving office. The former special counsel again said he believed that was the case.

Mueller has pointed to Department of Justice (DOJ) guidance that states a sitting president cannot be indicted as preventing his office from even considering whether to charge Trump with a crime.

Buck had criticized Mueller earlier on in his line of questioning over his handling of the report.

"Having reviewed your biography, it puzzles me why you handled your duties in this case the way you did," the Republican lawmaker said.

Buck hit the former special counsel for laying out instances where Trump may have committed obstruction of justice, but declining to make a decision on whether the president had actually committed obstruction.

"By listing the 10 factual situations and not reaching a conclusion about the merits of the case, you unfairly shifted the burden of proof to the president, forcing him to prove his innocence while denying him a legal forum to do so," Buck said.

Mueller said that because of the DOJ guidance, "one of the tools that a prosecutor would use is not there." And he said that his office "did not make that calculation" as to whether Trump had committed obstruction of justice.

"You made the decision on the Russian interference, you could have indicted the president on that and you made the decision on that," Buck said, pushing back. "But when it came to obstruction, you threw a bunch of stuff up against the wall to see what would stick. And that is fundamentally unfair."

"I would not agree with that characterization at all," Mueller replied.

Whether Trump would be charged with obstruction of justice were it not for the Justice Department guidance has been an area of focus for Democrats, some of whom are pushing to start impeachment proceedings against Trump in light of Mueller's findings.

Hundreds of former federal prosecutors and DOJ officials have signed on to a letter saying that they believe Trump would have been charged with a crime were it not for the guidance.