Up until now, the congressional committee investigating the 6 January attack on the Capitol was missing a key piece of the puzzle - the testimony of someone who could offer a first-hand account of the situation in the White House in the hours before and during the attack.
Cassidy Hutchinson, a former top aide to White House Chief of Staff Mark Meadows, filled in the blanks. And she has painted a devastating picture, including an allegation, which Trump denies, that he tried to grab the steering wheel of the car he was travelling in and wrestled with a Secret Service officer in an attempt to divert his motorcade to the Capitol, where his supporters were gathering.
A threat of violence ignored
Very early in the proceedings, the committee went to lengths to establish how the White House, and the president himself, knew that there was a very real threat of violence on 6 January - and did nothing to stop it.
Ms Hutchinson testified that Mr Meadows told her he thought, days before the attack, that things "might get real, real bad".
She spoke of how White House officials were warned of the potential for violence. And, in perhaps the most damning testimony so far, she said Donald Trump personally knew that members of the crowd at his morning rally near the White House were armed because they were being turned away by Secret Service officers - and directed them to the Capitol anyway.
"I don't [expletive] care that they have weapons. They're not here to hurt me," Ms Hutchinson said she heard the president say. "Let my people in. They can march to the Capitol from here."
A president enraged
Some of Ms Hutchinson's most damning testimony came second-hand, however. She recounted how a White House official told her that the president had insisted on travelling to the Capitol after his White House rally - something he said he would do during his speech. When he learned the motorcade was going back to the White House, he attempted to grab the steering wheel and wrestled with a Secret Service officer.
"I'm the [expletive] president," Trump said, according to Hutchinson. "Take me up to the Capitol now."
Since Ms Hutchinson's testimony, a source close to the Secret Service has told CBS News that both the agent and driver travelling in the car with Mr Trump were willing to testify under oath that the former president did not physically attack either of them and never attempted to grab the steering wheel.
Later in the day, Ms Hutchinson recounted hearing Mr Meadows say that, upon learning that rioters were calling for Vice-President Mike Pence to be hanged, Mr Trump expressed approval.
"He thinks Mike deserves it," Ms Hutchinson said she overheard her boss say. "He doesn't think they're doing anything wrong."
In a trial court, such evidence would be considered hearsay and treated with scepticism. In the hearing room, however, it was explosive - and will be used by the committee to pressure senior Trump officials who have so far refused to testify, like White House top lawyer Pat Cipollone, to come forward and either corroborate or refute her accounts.
"If you heard this testimony today and suddenly you remember things you couldn't previously recall, or you discover some courage you had hidden away somewhere, our doors remain open," committee chair Bennie Thompson said at the conclusion of the day's hearing.
A composed witness
The January 6th committee, with its surprise announcement of a mystery witness and new evidence unearthed, set a glaring spotlight on Ms Hutchinson during her in-person testimony on Tuesday.
For a 25-year-old who four years ago was a White House college intern, she held up to the pressure remarkably well.
She answered the committee's questions in a calm, methodical voice, noting how and under what circumstances she gained the information she was recounting. The committee made a point of showing how Ms Hutchinson's office was just a few doors down from the president's Oval Office and how she controlled access to Mr Meadows' office, giving her a prime position with which to witness - and, at times, overhear - conversations between key figures in the run-up to the Capitol attacks.
Her meticulous recollection of events and account suggest she may have kept a record of the events during her time at the White House or, at the very least, has an electronic record of texts and emails that supports her claims.
Donald Trump's rebuttal
As Ms Hutchinson was giving her at times damning account of the president's actions before and during the 6 January attack, Mr Trump took to his social media platform and began trying to undercut her claims.
Much of it was typical of the way he has responded to past critics, saying that he hardly knows Ms Hutchinson but hears "very negative" things about her. He called her a phoney and a "leaker" and suggested she was bitter because he didn't give her a job after leaving the White House.
He went on to deny many of the episodes Ms Hutchinson described and, once again, noted that he said in his rally speech that the crowd should march on the Capitol "peacefully".
It's always an open question whether any negative stories of Mr Trump's behaviour will dent his popularity among his supporters. Tuesday's testimony, and the five hearings before it, however, may remind some Republicans of the kind of chaos that frequently swirled around the Trump presidency and that, while he had some conservative accomplishments while in office, he also presided over his party losing both chambers of Congress and the White House.
Given that a potential 2024 opponent, Florida Governor Ron DeSantis, is rising in head-to-head-polls against Mr Trump, these hearings may have caused real damage to the former president's political power.