“How they were not ready for this today, I have no idea,” said Charles Ramsey, a former D.C. police chief. “They were overwhelmed, they did not have the resources. You have to be able to protect the Capitol. That is not OK.”

The Capitol Police did not answer the phone or email, or put out any statements about the incident.

Members of Congress demanded explanations as well. “We must investigate the security breach at the Capitol today,” said Representative Maxine Waters, a California Democrat, on Twitter. “I warned our Caucus and had an hour-long conversation with the Chief of Police 4 days ago. He assured me the terrorists would not be allowed on the plaza & Capitol secured.”

Representative Val Demings, a Florida Democrat, said in an interview with MSNBC on Wednesday night that the Capitol Police appeared understaffed. “It did not seem that they had a clear operational plan to really deal with” thousands of people who descended on the Capitol after Mr. Trump’s complaints of a “rigged election,” she said.