Inside the doors was the lieutenant, who, according to his account, had trained to deal with an active shooter but never a scenario like this, in which the Capitol was being overrun by large numbers of people. Calls for backup and reports of officers engaged punctuated the radio traffic.

The officer, according to his account, could not see the three uniformed officers outside and did not know they were there — he only described seeing a hallway full of oncoming people. The three officers had no visible shields or riot gear — two of them were not even wearing hats.

According to the lieutenant’s account, he did not know who among the rioters, if anyone, was armed. Nor could he see how far down the hall the crowd extended.

The lieutenant was also unaware, those briefed on his account said, that a tactical team from the Capitol Police was climbing the stairwell behind Ms. Babbitt, intending to reinforce the area and clear out the rioters.

As the team arrived, one of the three officers standing guard gave the word: “They’re ready to roll.”

The officers moved away from their post, leaving the doors unguarded for a crucial 30 seconds.