What makes me think it wasn't an extremist black is the reward offer. It seems completely unnecessary and another hook being added to the flier. An extremist wouldn't need a half million dollar payout to go out and doing something extreme. More importantly, if someone was extremist enough to put forth a flier with the intent of trying to get the prosecutor killed, they'd probably be extreme enough to do it themselves.
The dollar sign also feels like a poke at blacks for looking dumb.
The flier just feels a bit fake in its outrage. But it is possible a black extremist made it. But I'd hedge the other direction. I have been wrong in the past, so take my review with a chunk of salt.