I was able to find a video of obvious police agitators for my family member a few years ago (they had cop boots and were allowed to cross the cop line when the crowd turned on them), but it's been memory-holed. I can't find it after about 10-20 searches. If someone still had this, please link. The leader was screaming into a megaphone that the violent three were not a part of their group.
I know what you are talking about, but there have been so many police infiltrators of protests since then, that it's hard to recall the specific instance. It predates george floyd, and I think it was at the protest for Michael Brown when the police left him dying in the street for over an hour before even attempting to render aid, even stopping EMTs from attending him. It was one of the first big BLM protests that I remember.