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Robert Mueller, the official who led an investigation into alleged Russian interference in the 2016 US election, is to testify before Congress.
He will face questions at two lengthy hearings, with the first set to begin at 08:30 local time (12:30 GMT).
Mr Mueller has been reluctant to testify, but agreed to do so after two committees issued subpoenas.
His testimony comes three months after he released his findings on Donald Trump’s campaign and Russia.
His much-anticipated 448-page report said it had not established that the campaign criminally conspired with Moscow to influence the election.
But it did detail 10 instances where Mr Trump had possibly attempted to impede the investigation.
Mr Mueller will likely face questions on these instances of obstruction, alleged Russian election meddling, the behaviour of the FBI, and whether the president could have been charged with a crime.
What will happen?
Mr Mueller, a former FBI director, will appear before the House Judiciary Committee for around three hours. After a short break, he will then face two hours of questioning in front of the House Intelligence Committee.
The Democratic chairmen of both committees, Jerry Nadler and Adam Schiff respectively, will lead the questioning. Republican committee members will then ask questions of their own.
Although this is the first time Mr Mueller has answered questions publicly on the report, he is unlikely to drop any bombshells.
This is because he has vowed to stick closely to its contents. “The report is my testimony, ” he said in May. “I would not provide information beyond that which is already public in any appearance before Congress.”
Mr Mueller “will stick to the four walls of the report”, his spokesman clarified earlier this week.
What questions will be asked?
That depends on who is asking. Democrats and Republicans have very different expectations for these high-profile, televised, hearings.
On one hand, Democrats are expected to ask in greater detail about the instances where Mr Trump may have attempted to impede the investigation.
They will probably attempt to push Mr Mueller into stating outright whether the president could have been charged with obstruction, if not for the legal restrictions on prosecuting a sitting president.
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The relationship between Attorney General William Barr and Mr Mueller’s team will also come under scrutiny. Earlier this year, it emerged that Mr Mueller had written to Mr Barr and expressed frustration that the attorney general’s summary of his report had not captured the full context of its findings.
Democrats, analysts say, will ask about Mr Mueller’s widely reported frustrations with the Department of Justice.
Republicans, however, are likely to grill the former special counsel about the length and resources his almost two-year investigation used.
They are also expected to emphasise that the investigation – long branded a “partisan witch hunt” by Mr Trump – has ended and so the issue should be dropped.