Unless something more sinister emerges the simplest explanation is probably the best - there some cops who are far-right sympathisers and there are incompetently managed and organized Police forces. Both of those things are already demonstrably true and explain how the response quite well without introducing a grand conspiracy.
However, it is no secret that the FBI has long-reported (2006!) on white-supremecist infiltration of police forces[1] - this is not crazy talk. If someone joined the police as a rookie in 2006 to enforce their personal agenda, how far up the leadership hierarchy would they be now?
1. https://www.pbs.org/newshour/nation/fbi-white-supremacists-i...
This picture?
https://commons.m.wikimedia.org/wiki/File:Pence_posing_with_...
There is not enough information to come to either conclusion, but I would like to think the DC police leadership didn't/doesn't plumb those depths of incompetence. The public (and congress) deserves answers on what happened and why.