I don't know what the Shelton presence was in 1943.
I've heard that Peoria was considered an open town back then; meaning that mob organizations could operate here but none was supposed to try to take control.
I haven't heard any suggestions that it was domestic so the general motive had to be that he made the wrong person mad.
I suppose random is a possibility although extremely unlikely in my view.
The question is, why ditch the car in a place where it would be immediately noticed?