From this video:
Look at time stamp 0:20 to 0:26, I believe the back attacker is a foreign protester/agitator.
First, he dress differently than other masked protesters.
Second, he wear ski-type-mask/hood that cover his whole head(Hides his hair color). Nobody around wear that type of mask.
Third, he puts gloves before going in to attack from behind(hides finger print).
Fourth, His build(more muscular) and height(much taller) is different than of those other masked protesters. His skin tone is also whiter, which you can see when you grab the cop from behind.
And lastly, he stand there with ease, no talking, scream, eyes targeting the cop, puts on his gloves, moved in dexterity and attacked from behind. Very well trained.
Once he started the violent chaos, he disappeared from the scene.