In all fairness, based on an on-the-ground right wing source, what basically happened is that the "usual suspects" as we may call them (Probably there hoping there would be some shitstarting counterprotesters to brawl with) started to intimidate the police and pushed the rather negligable police presence backwards. This police then backed up into one direction; where the hooligans started to do the more vandal acts of breaching like bashing windows.

This entirely left open a path for the protestors to a different entrance at the final defensive police line, where the policemen probably figured their colleagues just let them through so they went like "Hey, good lads, feel free to enter. Behave though, and we still might to arrest you on federal trespassing into Congress but hey, thats Wednesday nowadays. Have fun!" and then proceeded to let the confused crowd of boomers and middle aged white people waltz in.

I stay with the point that if there was even a single legitimate terrorist in the crowd; we would've known. This situation could've been easily abused by anyone to go to one of the offices that doesn't connect to the window, which, mind you, are filled with wonderful oh-so-flammable paper and hardwood furniture, clog the fire alerts and then just leave a few ignited molotovs that would eventually explode. None of this happened.