"The department prepared in order to meet these challenges, but we did not do enough," said Yogananda Pittman, who took over when the ex-chief resigned.
Despite "strong potential for violence" the force did not adequately prepare for a "terrorist attack," she said.
Five people, including a Capitol Police officer, died in the riot.
Chief Pittman was speaking on Tuesday to the House Appropriations Committee, which oversees the police department's funding.