It's plausible, but he was getting ready to retire and didn't typically involve himself in investigative work so far as I know, so I'm not sure that he would have been doing anything related to his regular duties.
It could have been something personal. The obvious scenario that comes to mind is an affair with a married woman. The trouble with that scenario is that no other person was reported missing from the area around the same time as Ray, and if the woman was not killed, then she must have known what happened to Ray.
It could have been a revenge scenario where the woman was an accomplice. In that case, the perp would probably have been someone Ray put behind bars years beforehand who finally got out and who held a grudge.
It could also have been the drug traffickers from Ray's current case, but their motive would have been very weak. Ray was about to retire, and his replacement might have been even more aggressive than he was. Moreover, the death of a sitting D. A. would have shined a light on criminal activity in the county in a way that no organized crime outfit would want.