Introducing Hutchinson to the American people, Chairman Bennie Thompson asked Hutchinson to recall a typical day at the White House.

"When I moved over to the White House chief of staff's office with Mr. Meadows, when he became the fourth chief of staff, it's difficult to describe a typical day," she said.

Thompson established through a series of questions how Hutchinson's office was a five to 10-second walk from the Oval Office and that she regularly engaged with members of Congress and senior members of the Trump administration.

Vice chair Liz Cheney said Cassidy Hutchinson was in a "position to know a great deal about the happenings in the Trump White House."

"Today, you will hear Ms. Hutchinson relate certain first-hand observations about President Trump's conduct on Jan. 6," Cheney said in her opening statement. "You will also hear new information regarding the actions and statements of Mr. Trump's senior advisers that day, including his chief of staff Mark Meadows, and his White House counsel."

Cheney said information will also be released on what Trump and members of the White House knew about the potential for violence on Jan. 6.

Chair Bennie Thompson, D-Miss., convened the unexpected hearing shortly after 1 p.m. with Cassidy Hutchinson, a former top aide to Trump White House chief of staff Mark Meadows, taking her seat as members took the dais.

In brief opening remarks, he explained information that she had needed to be shared with the American people "immediately" and hailed her courage.

"In recent days, the select committee has obtained new information, dealing with what was going on in the White House on Jan. 6, and in the days prior. Specific, detailed information about what the former president and his top aides were doing and saying in those critical hours. Firsthand details of what transpired in the Office of the White House chief of staff," Thompson said.

"It hasn't always been easy to get that information, because the same people who drove the former president's pressure campaign to overturn the election are now trying to cover up the truth about January 6. But thanks to the courage of certain individuals, the truth won't be buried. The American people won't be left in the dark," he added. "Our witness today is Cassidy Hutchinson, she has embodied that courage."

With the nature of the hearing coming up with little notice, signaling urgency for the committee, reporters and cameras swarmed the witness table inside the Cannon Office Building ahead of Cassidy Hutchinson taking her seat.

Hutchinson entered the hearing room at 1 p.m. with members of the Jan. 6 committee.

ABC News Chief Washington Correspondent Jon Karl said sources have told him the hearing will be "Big —and disturbing."

The bombshell information the committee is unfolding through Cassidy Hutchinson's testimony is that the Trump administration and Trump himself knew about the potential for violence before Jan. 6.

"I recall hearing the word 'Oath Keeper' and hearing the word 'Proud Boys' closer to the planning of the January 6 rally when Mr. Giuliani would be around," Hutchinson said in a taped deposition played by Vice Chair Rep. Liz Cheney.

Cheney then displayed a Capitol Police bulletin on Jan. 3 warning, "targets of the pro-Trump supporters are not necessarily the counter-protesters as they were previously, but rather Congress itself is the target on the 6th."

Hutchinson also recalled receiving a call from then-national security adviser Robert O'Brien, after the Capitol Police bulletin, asking if he could speak with Meadows about the potential violence. She wasn't sure if that call ever happened.