Ever since the Mueller probe was launched in 2017, Trump has had a lot to say.

He has (so far) not tweeted since this morning's hearing started, but here are some of his past tweets on the issue.

Alabama Republican Congresswoman Martha Roby asks about how the redacted report was produced.

Mueller says that he worked with Attorney General William Barr to determine what to omit.

Mueller says that he never expected his report to be released publicly, and said it was produced just for the attorney general to review it.

Roby asks if Mueller's approach changed after it was announced that his report would be released to Congress and the general public.

"Can't get into that," Mueller replies.

Muller says that he attended "very few" of the nearly 500 interviews conducted by the special counsel's office.

Mueller confirms: President Trump resisted an attempt to question him in person.

Image source, Getty Images

Florida Democrat Representative Ted Deutch also focuses on McGahn and the obstruction of justice.

"The most important question I have for you today is: Why. Why did the president of the United States want you fired?"

Mueller says he can't answer that.

"Director Mueller you found evidence, as you lay out in your report, that the president wanted to fire you because you were investigating him for obstruction of justice, isn't that correct?," he asks.

"That's what it says in the report, yes," replies Mueller.

Image source, Getty Images

"That shouldn't happen in America," Deutch says. "The president ordered you fired. The White House Counsel knew it was wrong. The president knew it was wrong... but the president did it anyway.

"Anyone else who blatantly interfered with a criminal investigation like yours would be arrested and indicted on charges of obstruction of justice."

Deutch concludes: "You determined that you were barred from indicting a sitting president, we've already talked about that today. That is exactly why this committee must hold the president accountable."

For nearly two years the Trump-Russia affair has dominated front pages and mired the president's administration in conflict and controversy. But what is it exactly? How did it begin? And what did it find out?

Here's a breakdown on one of the most high-profile political inquiries in US history: what we know, what Mueller found, key players, and what's next.

Key questions answered

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A 2013 "puff piece" in the Washingtonian magazine stated that "you'd drop everything" if former FBI director James Comey - who was fired by Trump - rang, begins a visibly angry Republican Congressman Louie Gohmert.

He asks if they were friends, and Mueller says they were "business associates" before reluctantly saying "we were friends".

"Thank you, that was my question," says Gohmert.

"When did you first learn of Peter Strzok's animus towards the president?" Gohmert then asks, referring to the FBI investigator who was found to have exchanged anti-Trump text messages with Lisa Page, with whom he was having an affair.

Strzok was "swiftly reassigned", Mueller says, but Gohmert says he doubts that.

He accuses Mueller of "hiring dozens of people who hate" Trump to lead the investigation.

Democrat Hank Johnson of Georgia gets into Mueller's lack of conflicts of interest.

Mueller had none - per White House counsel Don McGahn and the Department of Justice ethics team - but the president "prodded" McGahn to see if Mueller could be removed over conflicts of interest, Johnson says.

After Trump was "warned about obstructing justice", Johnson says the president still talked about Mueller with McGahn, who "did not want to have a role in asking the attorney general to fire the special counsel".

Mueller refers Johnson to the report "and the way it is characterised in the report".

McGahn notably did not turn up to testify before Congress about the Mueller report in May.

Image source, Getty Images

Robert Mueller was given the powers of a US attorney and an order to look into Russian efforts to interfere in the election, links between Russia and the Trump campaign, and anything else that might arise from his inquiry.

So what other powers does a special counsel have, and what did Mueller do during his investigation?

Read more here.

Republican Steve Chabot is asking questions about the Steele Dossier, the controversial report that alleged links between Trump and Russia.

Mueller says he is not familiar" with Fusion GPS, the company that produced the Steele Dossier on Trump.

He describes questions about Fusion GPS as "outside his purview".

Democrat Steve Cohen of Tennessee asks about the president's reaction to the special counsel's appointment.

"He wasn't pleased," Cohen says, adding that the president was not happy when his attorney general at the time, Jeff Sessions, recused himself from overseeing the investigation.

Mueller confirms the details that are in the report.

He says it's true that the president asked Sessions to "unrecuse" himself, twice.

Does Mueller know of any times when the president expressed anger at Sessions?

Mueller says he'll "have to pass on that".

Need a refresher on the Trump-Sessions row? Watch below.

At the start of his testimony, the former special counsel offered a summary of his inquiry's key findings.

Jim Sensenbrenner, a Republican Congressman, asks that Mueller "speak more directly into the microphone" as he answers his questions about "prosecutorial judgement".

Why did we even have an investigation of Trump, if he could not be charged, asks Sensenbrenner?

We didn't know at the time how the investigation would unfold, Mueller says, adding that rules dictate that an investigation can continue even if there is no chance of a prosecution.

"Our mandate does not go to other ways of addressing conduct," Mueller said.

The Republican congressman says that's like a "fishing trip" for investigators.

When Mueller asks what document he was reading from, Sensenbrenner shoots back: "I'm reading from my question."

Democratic presidential hopefuls have called for the impeachment of President Trump as Mueller's testimony goes on.

Rep Jackson Lee asks: "Does obstruction of justice result in a lot of jail time?"

Mueller replies: "Yes, well, again, can you repeat the question just to make sure I have it accurate?"

"Does obstruction of justice warrant a lot of time in jail if you were convicted?" she repeats.

"Yes," comes the reply.

Sheila Jackson Lee, a Democrat from Texas, also chooses to focus on obstruction.

She asks about the president's efforts to "curtail the investigation".

"When you talk about the president's pattern of conduct that includes 10 possible acts of obstruction?"

Mueller directs her to the report.

"Is it true that the evidence does indicate that a thorough FBI investigation would uncover facts about the campaign and the president personally that the president could have understood to be crimes?"

Mueller says again he relies on the language of the report.

Republican allies of Trump in Congress are challenging Mueller's claim that Trump was not "exonerated" by his special investigation.

Ratcliffe adds that Donald Trump is not above the law, "but he damn well should not be below it".

Republican John Ratcliffe asks "about the DOJ policy or principle that sets forth a standard that a person is not exonerated if their criminal conduct is not conclusively determined".

"Can you give me an example other than Donald Trump where the Department of Justice determined that an investigated person was not exonerated?"

Mueller says no, but adds that this is a "unique" situation.

Ratcliffe responds that it was never Mueller's job.

"The special counsel's job - nowhere does it say you were to conclusively determine Donald Trump's innocence... or whether or not to exonerate him," he says.

"Volume two of this report was not authorised under the law to be written. It was written to a legal standard that does not exist."

The BBC's Tara McKelvey describes the feeling in the nation's capital as Mueller's testimony begins.