I hate to say it but it could have been nearly anyone who PUT the note where it was found.......a policeman or an "agent" or nearly anyone. This whole thing is one bad joke! It sure took any attention off the OKC-Muslim-Jayna Davis-Arlen Sphincter story and everything else. (By the way, the Sphincter punked out on an appearance he was to make last night on Pittsburgh TV to talk about OKC/Muslim connection. Single Bullet Sphincter-what a never-ending whore!)
Back to the shooter(s). I say, tell the guy, if he must continue, go out to Berserk-ley!
I think it'd be real tough for an unrelated third party to get into the area of the crime scene that night and leave the note, which leads us to your other possibility: a cop. This creep's ability to avoid the dragnet night after night together with the note sure argues for him having an inside line of some kind.