“The Capitol Police were unprepared for the sheer size of the protest,” said David Gomez, a retired FBI executive. Once the rioters barged into the Capitol, other federal law enforcement agencies were slow to respond, either out of deference to President Trump or because of a lack of experience with dealing with riots, which isn’t their primary mission, Mr. Gomez said. “Up until they breached the Capitol, the possibility existed that it was going to be a large protest that didn’t cross those barricades. Once they did that [law enforcement personnel] were overwhelmed and couldn’t respond quickly enough,” he said.