Throughout the two investigation, Donald Trump and his supports called the job Mueller was doing a witch hunt. News that were coming out about his administration colluding with Russia was called "Fake News".
During Muellers testimony, the Democrats asked the special counsel several questions and at the end of each one they asked "its a crime, right"? The response to each question was yes which confirms the level of criminality that was investigated.
In the end Mueller reiterated that if Donald J. Trump was not the President, he would have been arrested for breaking the law.