Robert Mueller has had a long career in the US justice system and has now testified to Congress about his probe into Russian meddling in the 2016 US election.
US President Donald Trump has declared that Robert Mueller's high-stakes testimony in Congress had been a win for the White House - and a "disaster" for his Democratic opponents.
"We had a very good day today," Mr Trump told reporters, once more slamming the special counsel's investigation into Russian election meddling as a "witch hunt."
"There was no defence to this ridiculous hoax," Mr Trump said.
"So this was a very big day for our country. This was a very big day for the Republican Party. And you could say it was a great day for me."
Asked about the fact Mr Mueller denied having exonerated him of obstructing the two-year probe into Russian election meddling, Mr Trump said that the special counsel "didn't have the right to exonerate."
During seven hours of nationally-televised hearings, Mr Mueller strongly denied Mr Trump's charge that his two-year investigation was a "witch hunt" led by anti-TrumpDemocrats.
Mr Trump dismissed Mr Mueller's performance before two congressional committees - during which he largely stuck to the script of his massive report - as "obviously not very good."
"He had a lot of problems," Mr Trump said. "But what he showed more than anything else is that this whole thing has been three years of embarrassment and waste of time for our country."
"This has been a disaster for the Democrats," he added.
On Wednesday, Mr Mueller, who spent 22 months investigating Russian interference in the 2016 US election and Mr Trump's conduct, appeared first before the House of Representatives Judiciary Committee.
The committee's Democratic chairman, Jerrold Nadler, praised Mr Mueller and said no one, including Mr Trump, is "above the law".
Mr Mueller did not respond to questions from reporters as he arrived, then was flanked by security personnel as he made his way toward the committee’s hearing room.
He is not expected to deliver any new bombshells, according to Democratic aides, but rather stick to the contents of his 448-page investigation report about the 22-month-long probe of Russian election meddling.
Mr Mueller plans to deliver an opening statement before taking questions.
Republicans are expected to object to the presence of Aaron Zebley, the former deputy special counsel who had day-to-day oversight of investigations in the inquiry, who will accompany Mr Mueller.
Mr Zebley will be present at the Judiciary hearing, according to Mr Mueller’s spokesman Jim Popkin and a House Judiciary staffer, and will be sworn in as a witness for the intelligence panel, according to an aide for that committee.
“This was specifically NOT agreed to,” Mr Trump wrote on Twitter before the hearing was set to begin.
Democrats hope the 74-year-old former FBI director will give the US public a compelling account of Russia’s sweeping interference, the Trump campaign’s readiness to accept help from Moscow and Mr Trump’s efforts to impede the Russia probe that Mr Mueller investigated as potential obstruction of justice.
Mr Trump, running for re-election in 2020, has blasted the Russia probe as a “witch hunt” and has repeatedly assailed the investigators including Mr Mueller.
“Seeing is believing in America. That report was voluminous. But most Americans didn’t read it. So they’ll see Mueller lay out the case,” said Representative Eric Swalwell, who sits on both the Judiciary and Intelligence panels.
Mr Mueller’s appearance could also be a turning point for Democrats on the question of impeachment.
Many liberal Democrats are pushing for an impeachment inquiry against Mr Trump, whose recent incendiary rhetoric about four congresswomen from racial minorities also has ignited a political firestorm.
Eighty-nine House Democrats, about 38 per cent of them, now want an impeachment inquiry against Mr Trump, according to a Reuters survey.
That is well short of the 218 votes needed to adopt articles of impeachment in the House, which would trigger a trial in the Senate on whether to remove Mr Trump.
But the number backing impeachment could swell if Mr Mueller’s testimony proves to be compelling.
“We hope Mueller’s testimony will be a watershed,” said Democratic Representative Sheila Jackson Lee, a Judiciary panel member.
The impeachment question has divided Democrats, with House Speaker Nancy Pelosi opposing such a move as politically risky for moderate Democrats on whose future the House Democratic majority depends.
Some Democrats prefer to remove Mr Trump from office the ordinary way: defeating him in next year’s election.
Mr Mueller, who had expressed reluctance to testify, agreed only after being subpoenaed.
The former federal prosecutor and US Marine Corps officer is also known for favouring one-word answers at hearings and could prove too taciturn to satisfy Democrats’ hopes of a forceful narrative.
“Let us listen, let us see where the facts will take us,” Ms Pelosi said of the hearings.
“We’ll see what happens after that.”