I dont know exactly who did that. CNN is reporting that it was the Garland Police Department. At the same time, other sources indicate that there were uhmmm..... "private security" guards of the expensive kind there. The men in the unmarked uniforms that I have seen in photos appear more like high end contract mercenaries than Police Department SWAT to me. At the same time, they were answering press questions which could mean police.
In either case, they clearly knew what they were doing. Though I avoid giving needless offense to anybody as it causes needless confrontations, if I was going out of my way to provoke a large group of people, they are the type of security that I would want around me.