I have stated this in other comments, but I feel it is extremely important. There is no rational way to attribute this to simple human error, given the overwhelming evidence of how protests on much more irrelevant topics, moments and locations are normally treated by US police.

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.