Theres a video of the police letting the protesters into the capitol. They opened the door and are standing to the side EDIT: I urge people to go watch the video. My opinion is that many walking in thought it was a protest and was just something the group was doing.
The bigger question is whether there was any deliberate decision to withhold necessary force --- in particular, whether the executive had any influence on the lack of Guard support or other measures not being taken.
A subsidiary question is whether there were any cops that were deliberately facilitating this. When so far outnumbered, 5-10 rogue police officers could make a huge difference in outcomes.