Didn’t the mayor of DC literally request that the feds NOT provide federal agents due to experiences with the BLM protests? She probably saved many lives with that decision. https://thehill.com/homenews/administration/532739-bowser-to...
The mayor's request was basically that the Feds not unilaterally start escalating and militarizing police efforts in her jurisdiction without her involvement. In hindsight, that does seem to have been the correct strategy for her police department, because all the violence seems to have been concentrated within the jurisdiction of a different police department.
They could have easily guarded the building itself. Close the doors and post 30 guards at each entrance?