Three rioters convicted on federal charges for participating in the Capitol attack appeared in court just hours ahead of the prime-time event and asked for mercy before federal judges deciding their punishments.

"I made one mistake in my life and I have immediately took responsibility for it," said Michael Daughtry, a gun store owner and former police officer who pleaded guilty to a misdemeanor charge this past March. "I apologize to the court for my indiscretion. But does a person not get to make at least one mistake in their entire life?"

The sentencing hearings just blocks away from the Capitol offer a noteworthy split-screen as lawmakers and their staff are in the midst of final preparations to put their investigation's findings on full display for the American people. Click here for more.

-ABC News' Alexander Mallin

Several police officers who defended the Capitol on Jan. 6 and widows of law enforcement members who died in the aftermath will be present at the hearing.

Among them are Erin Smith, the widow of Metropolitan Police Department officer Jeffrey Smith; Serena Liebengood, the widow of Capitol Police officer Howie Liebengood; Sandra Garza, partner of Capitol Police officer Brian Sicknick; Capitol Police officer Harry Dunn; Capitol Police Sgt. Aquilino Gonell; and Metropolitan Police Department officer Daniel Hodges.

Dunn told ABC News Congressional Correspondent Rachel Scott the hearing will be "triggering."

"I think about Jan. 6 daily and tonight we are going to find out stuff we didn't know," he said.

Garza told Scott she's preparing to painfully "relive the nightmare of the day." Her longtime partner, Officer Sicknick, suffered two strokes and died of natural causes one day after engaging with rioters.

"Everybody should watch the hearings because they need the truth of what happened that day," Garza said. "These are the facts -- it's important for them not to only hear the witnesses but see it again." She added, "There has to be some accountability, people are dead because of what happened."

Chairman Bennie Thompson will warn the American public of the ongoing threat from "those in this country who thirst for power" when the Jan. 6 committee soon kicks off a series of public hearings laying out its investigation, according to an opening statement released by the committee.

"So tonight, and over the next few weeks, we're going to remind you of the reality of what happened that day. But our work must do much more than just look backwards. Because our democracy remains in danger," Thompson is expected to say. "The conspiracy to thwart the will of the people is not over."

"January 6th and the lies that led to insurrection have put two and a half centuries of constitutional democracy at risk. The world is watching what we do here," read the excerpt.

One of the first officers injured on Jan. 6, U.S. Capitol Police officer Caroline Edwards, who suffered a traumatic brain injury after she was thrown to the ground by rioters pushing bike racks, will deliver her firsthand account before the committee in a matter of hours.

Documentary filmmaker Nick Quested, who followed the Proud Boys through Washington as members of the extremist group marched on the Capitol and clashed with law enforcement, is also scheduled to testify live.

ABC News exclusively obtained some of Quested's extraordinary material, showing how a group of Trump supporters at a presidential rally transformed into an angry mob that broke into the Capitol. Click here for more.