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."

Cassidy Hutchinson said she was "disgusted" by President Trump's Twitter post during the Capitol attack disparaging then-Vice President Mike Pence for not single-handedly rejecting Joe Biden's electoral victory.

"Mike Pence didn't have the courage to do what should have been done to protect our Country and our Constitution, giving States a chance to certify a corrected set of facts, not the fraudulent or inaccurate ones which they were asked to previously certify. USA demands the truth!" Trump tweeted.

Hutchinson recalled "feeling frustrated, disappointed, and really -- it felt personal. I was really saddened. As an American, I was disgusted. It was unpatriotic. It was un-American."

Matthew Pottinger, who was then serving as the deputy national security adviser, told the committee in previous testimony, it said, that he decided to quit because of what Trump said in that social media post.

"I read that tweet, and made a decision at that moment to resign," Pottinger said. "That's where I knew that I was leaving that day, once I read that tweet."