I was told the cops let the protestors in
Yeah, that seems like a weird and dangerous talking point that circulated quickly. Capital police were incredibly outnumbered and probably outgunned from the get-go, there was no winning. For the officers on the ground, it was definitely a matter of strategic where/how to make a stand to keep the politicians and themselves safe.
That being said, the compete inadequacy of the decision makers to foresee and stop it from ever getting out of hand is seriously troubling.