Thinking about it more I don't think it makes sense for Paul to be dirty at both ends, finding the body and matching the watch, because it works best if the people who orchestrated each are different people working against each other rather than the same.
Killing Kasper wasn't just about f**king with Frank, if it was then there was no reason to place Kasper's corpse strategically where the mayor couldn't just quash it, indeed if it was just about Frank they could have just disappeared the body (and we get those scenes of the murderer driving Kasper out there, suggesting it's all very deliberate). It obviously does have a lot to do with Frank, seeing as they murdered his henchman too, but Frank isn't everything. The murderer wanted the investigation, but as far as we know the investigation hasn't achieved anything yet so why would they be the same people who shut it down?
Unless it has achieved something offscreen, Ray told Ani the investigation was to shake down the mayor, and maybe it has somewhat worked and the mayor has come to the party with whoever is pulling the strings on Ani (is it the state that black woman is working for? Is she a state prosecutor?) and so it was time to shut everything down.
Watching it again, they go out of their way to show the lazy cop watching Paul wasn't in on it. It was his informant that gave them the location and he was watching the place but when they show up he suggests just waiting for the baddies to come outside, and then when shit goes down they show his reaction first and it doesn't look like he's expecting shit. The higher up Vinci cop asked if they needed all these people for the raid at the station, maybe something in that.
