But he also noted, "if we had had confidence that the president clearly did not commit a crime, we would have said so." Trump reacted to Mueller's comments less than an hour later, tweeting: "The case is closed! Tweet Trump has repeatedly denied any wrongdoing by himself or members of his campaign, and has railed against Mueller's investigation, calling it a "witch hunt." However, Mueller noted that there was another avenue for dealing with a sitting president who broke the law. He said that the internal Justice Department opinion barring the prosecution of a president also "says that the Constitution requires a process other than the criminal justice system to formally accuse a sitting president of wrongdoing." That "process," which Mueller did not name, is impeachment.

U.S. Special Counsel Robert Mueller makes a statement on his investigation into Russian interference in the 2016 U.S. presidential election at the Justice Department in Washington, May 29, 2019. Jim Bourg | Reuters

A growing number of Democrats in the House are calling for an impeachment inquiry because of the findings of Mueller's report, and Trump's stonewalling of their demand for testimony from people connected to him, and documents. Mueller suggested that if he is forced to testify before Congress about his investigation — as Democrats are insisting — he "would not go beyond the findings in our report." "The report is my testimony. I would not provide information beyond what is already public," Mueller said, referring to calls to appear before Congress. "I am making that decision myself. Nobody has told me whether I can or should testify or speak further about this matter," said Mueller, who announced at the event that he was formally resigning and returning to private life. His findings included his determination that there was a sustained, aggressive effort by Russian agents to use social media and computer hacking to help sway voters to Trump over Democratic contender Hillary Clinton. "Russian intelligence officers who are part of the Russian military launched a concerted attack on our political system," Mueller said. But he noted that he also had found that there was not sufficient evidence to charge people affiliated with the Trump campaign with conspiring with Russian agents to affect the election.