Public Enemy's Prof. Griff: I Don't Know Dallas Cop Killer!!

Public Enemy's Professor Griff is shutting down rumors he and Dallas shooter Micah Johnson were pals ... saying a photo of the two shaking hands was only a one time thing.

Social media exploded with the image Friday after the pic was uncovered on Johnson's Facebook page. Griff tells TMZ he never knew Johnson personally and was shocked to see his own face pop up following the shooting.

Griff says he's been bombarded with vociemails and other messages from people threatening to kill him and his family since the pic surfaced.

And get this ... Griff tells us shortly after the image came up, he was contacted by his local cops for an interview. Griff says once he told the cops he only had the one chance encounter with Johnson, they let him off the hook.