Standing guard outside the doors were three Capitol Police officers. The crowd hurled insults at them and punched the glass just inches from their heads. To the right, at the top of a stairwell, stood a man in a suit with an earpiece, identified by a person familiar with congressional security as an unarmed member of the House sergeant-at-arms staff.

Near the front was Ms. Babbitt, 35, who had served 14 years in the Air Force and was an enthusiastic supporter of President Donald J. Trump. Her social media feed was filled with QAnon conspiracy theories.

One man in the crowd, David Charles Mish Jr. of Wisconsin, told an investigator later that Ms. Babbitt was telling the police officers, “Just open the door. They’re not gonna stop,” according to an affidavit.