Unlike many of their fellow Trump supporters, the Proud Boys did not show up for Mr. Trump’s address near the White House on Jan. 6. Instead, they assembled at the Washington Monument around 10 a.m., according to one of the committee’s live witnesses, Nick Quested, a documentary filmmaker who was embedded with the group that day.

About 200 members of the group gathered that morning, Mr. Quested said, and marched through the city streets, arriving at the Peace Circle on the west side of the Capitol at shortly before 1 p.m., just minutes before lawmakers inside the building were set to begin a ceremonial proceeding to certify the election. Mr. Quested said he was surprised by the Proud Boys’ anger and profanity as the larger crowd around them turned, as he put it, “from protesters to rioters to insurrectionists.”

Another witness at the Peace Circle was Officer Edwards, who testified about being in the middle of the first breach of the Capitol grounds. Ms. Edwards described how a Proud Boys leader named Joseph Biggs encouraged another man to approach the bike rack barricade where she was posted. That man, Ryan Samsel, she said, pushed the bike rack over, causing her to hit her head and lose consciousness.

But before she blacked out, Ms. Edwards recalled seeing “a war scene” playing out in front of her. Police officers were bleeding and throwing up, she recalled.

“It was carnage,” she said. “It was chaos.”

Reprising the theme of the police standing up to defend democracy that day while Mr. Trump did nothing to call off the mob he had incited, Representative Bennie Thompson, Democrat of Mississippi and the chairman of the committee, apologized to Ms. Edwards for the ordeal she suffered.

“It is unfortunate that you had to defend the Capitol from fellow Americans,” Mr. Thompson said.

Even as all of this was unfolding, Mr. Trump, watching from the White House, believed his supporters in the mob were “doing what they should be doing,” Ms. Cheney said.