LIVE: Mueller testifies on Capitol Hill at blockbuster hearing

Mueller began his opening statement by establishing that his investigation was conducted thoroughly and fairly.

"My staff and I carried out this assignment with that critical objective in mind: to work quietly, thoroughly, and with integrity so that the public would have full confidence in the outcome," he said.

Georgia Rep. Doug Collins, the ranking member on the committee, hinted that Republicans will use the hearing to question Mueller about the origins of the Russia investigation.

The DOJ's inspector general is also currently investigating the matter, and Attorney General William Barr said earlier this year that the results of that investigation will be released sometime this summer.

"Those results will be released, and we will need to learn from them to ensure the government's intelligence and law enforcement powers are never again turned on a private citizen or political candidate as the result of the political leanings of a handful of FBI agents," Collins said.