Rep. Ken Buck, R-Colo, a former prosecutor, questioned former special council Robert Mueller on Wednesday, with a heavy focus on the issue of obstruction of justice.

The lawmaker thanked Mueller for his service to the country, which included leading a rifle platoon in Vietnam, before saying because of this “it puzzles me why you handled your duties in this case the way you did.”

Speaking of fair treatment and citing Supreme Court Justice George Sutherland to say a prosecutor can strike hard blows, but is not at liberty to strike foul ones, Buck noted the ten points Mueller laid out in his report where Trump may have obstructed justice and said by not reaching a conclusion on their merits, “You unfairly shifted the burden of proof to the president, forcing him to prove his innocence.”

“I’ve never heard of a prosecutor declining a case and then holding a press conference to talk about the defendant,” Buck then said, referencing the press conference Mueller held last month.

The Republican lawmaker criticized Mueller for disregarding his legal duty to either prosecute or decline charges.

He questioned the statutes the special counsel used to support the ten instances, citing several occasions were they were incorrectly applied.

“The ethical rules require that a prosecutor have a reasonable probability of conviction to bring a charge, is that correct?” Buck asked.

Mueller replied that this was generally accurate.

After they agreed that Mueller found no evidence to suggest conspiracy with Russia in regard to the 2016 election, Buck then asked, “[w]as there sufficient evidence to convict President Trump, or anyone else, with obstruction of justice?”

“We could not make that calculation,” Mueller answered, citing his inability to indict a sitting president.

Buck stressed that Mueller made a decision on conspiracy, which was an indictable offense, yet did not do so on obstruction.

“When it comes to obstruction, you threw a bunch of stuff up on the wall to see what would stick,” the congressman said. “And that is fundamentally unfair.”

“You believe that you could charge the president of the United States with obstruction of justice after he left office?”

“Yes,” Mueller replied.

But when Buck asked him again if he could do so under the ethical standards, Mueller admitted that he did not look at them.