Mueller appeared at separate hearings held by the House judiciary and intelligence committees to testify about his 448-page report on the Justice Department's probe into Russian meddling in the 2016 election. The investigation confirmed Russia interfered but did not find evidence the Trump campaign colluded with Moscow.
Lawmakers focused on areas of the report detailing ways in which Trump and his close aides may have attempted to obstruct the Russia investigation. Mueller refused to exonerate Trump of obstruction.
Judiciary committee Chairman Jerrold Nadler said Trump publicly attacked the the special counsel investigation, urged against witness cooperation and wanted Mueller fired.
"Any other person who had acted in this way would have been charged with crimes," Nadler told Mueller. "In this nation, not even the president is above the law. There must be accountability for the conduct described in your report, especially as it related to the president."
Mueller said 30 defendants were charged with crimes, including some members of the Russian military, but emphasized a sitting president can't be indicted.
"Could you charge the president with a crime after he left office?" Rep. Ken Buck, R-Colo., asked, to which Mueller answered, "Yes."
"We focused on whether the evidence was sufficient to charge any member of the campaign with taking part in a criminal conspiracy," Mueller added. "And it was not. We decided we would not make a determination on whether the president committed a crime. That was our decision then and it remains our position today."
Nadler asked Mueller whether the report totally exonerated Trump. Mueller said "no," adding that Trump refused to be interviewed.
Rep. Ted Deutch, D-Fla., noted Trump wanted to fire Mueller to throw off the inquiry, citing statements to former White House counsel Don McGahn.
"The president wanted to fire you because you were investigating him for obstruction of justice," Deutch said.
Mueller acknowledged that's what the report said.
"Director Mueller, that shouldn't happen in America," Deutch continued. "No president should be able to escape investigation by abusing his power but that's what you testified to in your report ... The White House counsel knew it was wrong, the president knew it was wrong ... The president did it anyway. Anyone else who blatantly interfered with a criminal investigation like yours would be arrested and charged with obstruction of justice."
Rep. Zoe Lofgren, D-Calif., said the report shows 126 contacts between Russians and Trump campaign officials, and outlines the Russian campaign used advertisements and propaganda to influence the U.S. election.
"First, the Russians wanted Trump to win," Lofgren said. "Second, the Russians went on a sweeping cyber influence campaign. The Russians hacked the [Democratic National Committee] and they got the Democratic game plan for the election.
"They were armed with inside information that they had stolen through hacking from the DNC and that they had been given by the Trump campaign from the chairman Mr. [Paul] Manafort."
Asked by Rep. Cedric Richmond, D-La., whether it was "fair to say" Trump tried to protect himself by asking staff to falsify records, Mueller answered in the affirmative.
"I would say that's generally a summary," he said.
Asked if Trump intended to "hamper the investigation," Mueller referred Richmond to the report.
"This hearing has been very helpful to this committee as it exercises its constitutional duty where the recommended articles of impeachment against the president," Rep. Veronica Escobar, D-Texas, said.
House Republicans on the panel attempted to defend Trump and discredit Mueller. Rep. Doug Collins, R-Ga., said Trump didn't shut down the investigation "because he knew he was innocent."
Testimony before the House intelligence committee
Mueller shifted his testimony to the House intelligence committee Wednesday afternoon.
House intelligence committee Chairman Adam Schiff, D-Calif., said the Trump campaign "welcomed" Russian interference and "built Russian meddling into their strategy" before detailing hush money payments. He said Trump "didn't publicly shun" help from Russia and encouraged and made "full use of it."
"Putin wanted relief from U.S. economic sanctions imposed in the wake of Russia's invasion of Ukraine and over human rights violations."
Rep. Jim Himes, D-Conn., asked Mueller whether candidates should notify the FBI if they receive an offer of assistance, including the disclosure of damaging information on an opponent. Mueller answered "yes" and said using such information would be a crime.
Rep. Peter Welch, D-Vt., expressed concern that future candidates may not report foreign influence to authorities in wake of this investigation.
Mueller said he also fears that outcome and said informing U.S. intelligence agencies should be the "first line of defense" against such influence.
"I hope this is not the new normal, but I fear it is," he said.
Rep. Mike Turner, R-Ohio, said Trump's statements about being "exonerated" by the report are misleading because neither the attorney general nor special counsel have authority to exonerate.
"You have no more power to declare him exonerated than you have the power to declare him [CNN anchor] Anderson Cooper," Turner said.
Mueller clarified statements made during his earlier testimony saying he did not intend to state that he did not indict the president due to guidance from the Office of Legal Counsel, but rather that his team did not make a decision either way due to the OLC's guidance.
Additionally, after saying he believes Trump could be charged with obstruction of justice after leaving office, Mueller said he was not sure what would happen if Trump remains in office beyond the statute of limitations for the case.
Mueller scolded Trump's praise of WikiLeaks in response to Rep. Mike Quigley, D-Ill., reading quotes from the president including, "This WikiLeaks is like a treasure trove" and "Boy, I love reading those WikiLeaks."
"'Problematic' is an understatement in terms of what it displays in terms of giving some hope or some boost to what is and should be illegal activity," Mueller said.
He also referred to an exchange between Donald Trump, Jr. and WikiLeaks as "disturbing and also subject to investigation."
Mueller revealed that his team negotiated with Trump about a possible interview for "a little over a year," before deciding not to subpoena him to prevent dragging out the investigation.
"When we were almost toward the end of our investigation and we had little success in pushing to get the interview of the president, we decided that we did not want to exercise the subpoena powers because of the necessity of expediting the end of the investigation," he said.
Rep. Will Hurd, R-Texas, asked Mueller if he believed the subject of the investigation was a single attempt by Russia to interfere in the elections, to which the former special counsel responded that it is an ongoing effort that is likely to affect the 2020 elections.
"It wasn't a single attempt," Mueller said. "They're doing it as we sit here. And they expect to do it during the next campaign."
Trump pleased with results of Mueller's testimony
Trump said Wednesday was a "very good day," slamming the investigation at the center of Mueller's testimony as a "hoax" and "phony stuff."
"TRUTH IS A FORCE OF NATURE!" Trump wrote on Twitter at the conclusion of the hearings.
He also praised House Republicans for their questioning during the hearing.
"Republicans that defended something and defended something very powerful, very powerful, very important because they were really defending our country more than anything else," Trump said of the hearings. "They were warriors and they've been warriors for a long time. And everybody knew it was a hoax, especially the Democrats."
Trump criticized Mueller's performance during the testimony, describing it as "not very good."
"He had a lot of problems, but what he showed more than anything else is that this whole thing has been three years of embarassment and a waste of time for our country," Trump said.
Democrats still unsure about impeachment
"My position has always been whatever decision we made in that regard would have to be done with our strongest possible hand and we still have some outstanding matters in the courts," she said. "It's about Congress, the Constitution and the courts and we are fighting the president in the courts."
Rep. Elijah Cummings, D-Md., described Wednesday's hearings as "a critical moment in our country's history."
"They're going to ask the question, 'What did you do when we had a president who knew the rules and knew that our founding fathers had done a great job of creating a Constitution and had put in the guardrails but never anticipated that we would have a president that would just throw away the guardrails?" he said.
Schiff said Mueller's testimony indicates that the United States is in a "vulnerable" position going into the 2020 presidential election.
"Although we must do everything we can to harden our election defenses to make sure there are paper trails, to make sure that we deter and disrupt any kind of Russian intervention, but we cannot control that completely, but we can control what we do," he said.