The bottom line is that the police appeared to be totally understaffed even though the protests had been announced for weeks and were entirely predictable.
What we have seen last night was an attempt at mob rule, and it was almost obviously allowed by the police force whose duty it is to safeguard the security of the nation's most powerful institutions.
This doesn't mean that the Capitol building should be turned into a fort. But there is a vast distance between being open in general on one hand, and allowing a crowd of people whose stated intent is to overthrow the legally confirmed the results of the presidential election to amass on the steps of Congress and to breach the building while the legislative is in session confirming those results at the federal level.
The conspiratorial one is that it was deliberate. The question in that case is on what level (leadership, or on the ground). I hope investigations will show this. Who raised the issue of too little staff? Who denied reinforcements? Was there a massive number of officers calling in sick that morning? Etc etc.
There can be no reasonable doubt that this was allowed to play out, with the few policemen stationed there being openly friendly with the crowd by some reports. The level where the decision to allow the protest to attack Congress was taken is of course up to the courts to investigate, but I feel it is disingenuous to claim that this isn't the most reasonable conclusion by far.