DR insignia? Doctor in an ER?
The shooter is Klantifa member Maxwell Yearick. He was arrested in 2016 for assaulting a cop at a Trump rally.
https://www.wtae.com/article/pittsburgh-donald-trump-protesters-sentenced-to-jail-time/9261199
The shooter appears to be wearing one of these shirts. I’m not sure what significance, if any, there might be.
https://www.bunkerbranding.com/products/military-green-demolitia-t-shirt?variant=8137045639213