The capital is a public building. Someone would have to order them to lock the doors.
The standing orders are to lock the doors after hours. Further, most federal locations have a policy to close the location i.e. lockdown when there is a riot.
Someone had to order the police to let them in. Who?
Most entrances were probably limited to officeholders and staffers.
There would be a public entrance for lobbyists, delivery people and tradespeople, all subject to security screening.
In normal times, there would also be a tourist entrance with security screening.
I don’t think the public could just walk in since 9/11.