WASHINGTON — Robert Mueller told Congress on Wednesday that his investigation did not exonerate President Donald Trump of wrongdoing and found that Russia worked to boost his election in a "sweeping and systematic fashion" as the former special counsel defended his nearly two-year probe.

"It is not a witch hunt," Mueller said after Intelligence Committee Chairman Adam Schiff, D-Calif., noted that the president had often condemned the probe as just that.

It was a rare flash of emotion from Mueller, who otherwise proved to be taciturn, using his frequently short answers during back-to-back appearance before the House Judiciary and Intelligence committees to confirm a number of damaging facts contained in the report he completed this spring.

Offering little new information, Mueller declined to answer questions at least 200 times, according to an NBC News tally, and he wouldn't read out loud from his report, as lawmakers requested, instead urging them to do it instead. He often said questions were beyond the "purview" of his probe and was wary of saying much that could be construed as opinion.

Mueller's highly anticipated testimony began in the morning with the Judiciary Committee when Chairman Jerry Nadler, D-N.Y., asked the former special counsel if his investigation had indeed cleared Trump, as the president has often claimed.

"No," Mueller answered flatly.

The former special counsel, who testified under oath, also detailed why his team didn't decide one way or another whether Trump could be criminally charged, despite some evidence that the White House might have attempted to obstruct the investigation.

"Based on Justice Department policy and principles of fairness, we decided we would not make a determination as to whether the president committed a crime," Mueller said, referring to the longstanding policy set by the department's Office of Legal Counsel against indicting a sitting president.

The questioning resumed in the afternoon before the Intelligence Committee, whose members delved deeper into Russia's interference in the 2016 presidential election, which Mueller appeared more willing to discuss.

"Over the course of my career, I’ve seen a number of challenges to our democracy," Mueller said. "The Russian government's effort to interfere in our election is among the most serious."

And he warned that Russia's efforts to disrupt the U.S. democracy have not stopped and that much more needed to be done to confront the challenge Moscow — something Trump has often been reluctant to do as he hopes to usher in warmer relations with President Vladimir Putin.

"They're doing it as we sit here," Mueller said of Russian subterfuge. "And they expect to do it during the next campaign."

The former special counsel also criticized Trump for repeatedly praising Wikileaks — "we love Wikileaks," he said on the campaign trail in 2016 — after it published hacked Democratic emails that were later determined to have been stolen by Russia.

"Problematic is an understatement," Mueller said, "in terms of giving some hope or some boost to what is and should be illegal activity."

Download the NBC News app for full coverage of Mueller's testimony.

Mueller, who arrived on Capitol Hill shortly before 8 a.m. for the day of hearings, told lawmakers on both committees he would not stray beyond the findings of his report, but Democrats hoped his testimony will make its findings more accessible to millions of Americans who may not have read the document or have tuned out earlier news coverage.

Nadler kicked off the Judiciary hearing with his opening remarks addressed directly to Mueller, setting the tone for members of both parties who, throughout the day, would often begin their questions with a strongly worded statement that Mueller often had no interest validating.

"Among the most shocking of these incidents (in Mueller's report), President Trump ordered his White House counsel to have you fired and then to lie and deny that it ever happened; he ordered his former campaign manager to convince the recused attorney general to step in and limit your work; and he attempted to prevent witnesses from cooperating with your investigation," Nadler said.

"Although department policy barred you from indicting the president for this conduct, you made clear that he is not exonerated," Nadler added. "Any other person who acted this way would have been charged with a crime. And in this nation, not even the president is above the law."

Under questioning from Rep. Cedric Richmond, D-La., Mueller agreed that it was "correct" to say that Trump asked former White House Counsel Don McGahn to is misrepresent facts he knew to be accurate to defend the president.

"So it's fair to say the president tried to protect himself by asking staff to falsify records relevant to an ongoing investigation?" Richmond asked.

"I would say that's generally the summary," Mueller agreed.

Republicans followed Trump's lead in trying to discredit Mueller's probe and used their allotted questioning time during the hearing to sow doubts about the probe's origins, alleging that the special counsel overreached and asserting anti-Trump bias among the investigators.

Rep. Doug Collins of Georgia, the ranking Republican on the Judiciary Committee, used his opening statement to say the investigation was started based on "baseless gossip."

"The president's attitude towards the investigation was understandably negative," Collins said. "The president knew he was innocent."

Rep. John Ratcliffe, R-Texas, a former U.S. attorney, slammed Mueller for overstepping the bounds of his appointment by writing the second volume of his report, which focused on Trump's alleged efforts to stymie the investigation but ultimately did not charge him for it.

Ratcliffe said that portion of the report was essentially an attempt to smear Trump and violated the principle of innocent unless proven guilty.

"I agree with the chairman this morning when he said Donald Trump is not above the law. He's not," Ratcliffe said. "But he damn sure shouldn't be below the law, which is where volume 2 of this report puts him."

Rep. Louie Gohmert, R-Texas, a former judge, pressed Mueller on why he hired people who "hated Trump" as part of his investigative team, referring to former senior FBI agent Peter Strzok and FBI lawyer Lisa Page, who were secretly having an affair and exchanging messages critical of the president.

Mueller did not object to Gohmert's characterization of their feelings about Trump, but said he had no way of knowing about that when he hired them. Mueller said he"acted swiftly" to remove them once the issue came to light.

Strzok and Page have become central characters in Republicans' alternative storyline of the Mueller probe, which they say grew out of an opposition research investigation funded by anti-Trump Republicans and the Democratic National Committee and conducted by a firm called Fusion GPS and its agent, former MI6 Christopher Steele. Mueller, however, said he was not "not familiar" with Fusion GPS and could not comment on the origins of his probe.

Trump repeatedly attacked Mueller on Twitter and suggests Democrats backfired by putting so much stock in such a reluctant witness.

I would like to thank the Democrats for holding this morning’s hearing. Now, after 3 hours, Robert Mueller has to subject himself to #ShiftySchiff - an Embarrassment to our Country! — Donald J. Trump (@realDonaldTrump) July 24, 2019

Multiple sources familiar with Trump's thinking characterize him as annoyed but not overly enraged ahead of the former special counsel's testimony.

The president, on one hand, sees the prospect of Democrats overreaching on impeachment post-Mueller as a political winner, but he still finds it "incredibly annoying" and would rather the page be turned on this chapter of his administration, the sources said.

The president is irritated he still has to deal with Mueller more than two years after the special counsel investigation started. There's a "here we go again" exasperation inside the White House, with one person describing a "battle-hardened" atmosphere at this point.