Warrick Brown was doing his own investigation into a mob boss who was connected to a higher up in the department. He got too close and the crooked cop (who turned out to be the "under sherif" - not sure what that is) blew Warrick away in an alley.
The back story is that Warrick was framed for the murder of the mob boss by another crooked cop, because Warrick was starting to figure it all out. The CSI team was able to exonerate Warrick and figure out who is was that framed Warrick.
Bottom line, two crooked cops and a mob boss - all dead now after last night's show - brought down by Warrick and Katherine Willows and the rest of the CSI team.
