Robert Mueller is notorious for saying as little as possible. (AFP photo)

Robert Mueller said Wednesday he did not exonerate Donald Trump in his two-year Russian meddling investigation but also told lawmakers he will not say if the president committed a crime, as he launched into a marathon day of congressional testimony.

Three months after releasing the final report on his probe into the 2016 election, much of the American public remains unclear about the former special counsel's findings on whether Trump criminally obstructed justice and whether his campaign colluded with Russians.

With Trump declaring he was "exonerated" in the probe and Democrats saying the report supplied ample evidence for impeachment, the notoriously taciturn Mueller answered questions for the first time in front of two separate, Democrat-run committees in the House of Representatives.

Mueller -- who made a few public remarks at a press conference on May but did not take questions and has resisted testifying -- reiterated that he would "stay within the text" of his report, and was visibly uncomfortable once the question period began.

"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. That was our decision then and it remains our decision today," Mueller said in his opening remarks.

But when asked at the start of the hearing before the House Judiciary Committee if his report completely cleared the president of wrongdoing, Mueller said "no."

"The president was not exculpated for the acts he allegedly committed," Mueller said, adding that it was "true" that Trump could be prosecuted for obstruction of justice after he leaves office.

As the questioning volleyed back and forth from Democratic to Republican committee members, the tension mounted and ebbed. Mueller offered many one-word answers and repeatedly referred lawmakers back to the report.

Trump insisted this week he would not watch the nationally-televised testimony but half an hour before it was to start, he let loose with an angry Twitter tirade, complaining among other things about the fact Mueller will testify with a top aide sitting next to him.

"This was specifically NOT agreed to, and I would NEVER have agreed to it. The Greatest Witch Hunt in U.S. history, by far!" Trump wrote in one of a half dozen tweets.

The cryptic legalese used in Mueller's report allowed the president and his allies to claim the investigation was a politically charged witch hunt that found nothing of substance.

"NO COLLUSION, NO OBSTRUCTION!" Trump tweeted again on Tuesday.

Democrats nevertheless hoped Mueller would make clear to the public why he did not bring charges despite damning evidence outlined in the 448-page report -- and potentially offer explanations of Trump's conduct that would damage him in the run-up to the 2020 election.

"Every American deserves to know the crimes and corruption exposed in the Mueller Report," said Democratic Representative Ted Lieu.

Collusion, obstruction alleged

Mueller's report documents extensive contacts between the Trump campaign and Russians, including attempts to cooperate or collude, neither of which is a specific crime.

Mueller ruled in the end that there was not enough evidence to recommend charges of criminal conspiracy, the main legal charge he could make use of.

The veteran prosecutor and former FBI director also laid out in detail 10 instances when Trump allegedly tried to obstruct the investigation.

But he said he was prevented from recommending charges against Trump because Justice Department rules prohibited him from lodging criminal charges against a sitting president. He reiterated that position in Wednesday's testimony.

That left it to Congress to determine whether Trump committed a crime, and a minority of Democrats are pressing for the House to impeach the president.

Justice Department rules

Mueller, veteran of some 88 trips to Capitol Hill over a long career in government, is notorious for saying as little as possible.

But he also made clear he thinks the American public does not fully understand the extent and consequences of Russia's campaign to meddle in the election, which the president has repeatedly refused to acknowledge.

Trump's Justice Department meanwhile weighed in late Monday with a statement demanding that Mueller stick only to what he wrote in his report, in hopes of containing any fallout from his testimony.

"It's incredibly arrogant of the department to try to instruct him on what to say," Jerry Nadler, the chairman of the House Judiciary Committee, said on CNN Tuesday.

"It's part of the ongoing coverup by the administration to keep information away from the American people."