Now, to expand on the possible foul play scenario, the fact that so many of these detectives and family members are hinting at Clint knowing something, I do think he's a possible person of interest and is either currently or was on the police's radar at one time.
We don't really know what their relationship was like outside of hearsay stuff from people who knew them, but there's certainly a possibility that Brian and Clint were involved in some kind of dispute or altercation after their night out and something terrible happened.
For the record, I don't believe this is actually what happened, I think it's more likely that Brian was drunk, stumbling around in the early morning hours in a bad part of town, somebody thought he was an easy target because of his condition and decided to rob him, something got out of hand, and he was killed. His body could be anywhere near the area, or it could have been loaded into a vehicle and driven to another state for all we know.