The top officers of the Capitol Police and Metropolitan Police have been denying they had any idea there would be violence at the protests. As various commentators have been pointing out, this is absolutely absurd. The plans for violence were posted on the Internet in public forums. Moreover, one of the top Capitol Police officers was directly warned by a congress member:

https://www.lamag.com/citythinkblog/maxine-waters-interview-donald-trump/

The Capitol Police union has bitterly accused their leadership of failure and has asked for heads to roll.