In the Season 9 premiere, "For Warrick", Grissom found Warrick bleeding to death in his car; Warrick tried to tell Grissom that McKeen shot him, but was deterred by McKeen's brandishing of his sidearm, and died in Grissom's arms.

Why did they kill off Warrick on CSI?

As Warrick Brown, Gary Dourdan was similarly on CSI from the beginning. ... Unlike his co-stars, however, Dourdan didn't return to the show later down the road. That's because his character died during the season nine premiere. As it was reported back in 2008, it came down to contract negotiations.

Why does Gil Grissom leave CSI?

Why William Petersen Left CSI

In short, Petersen left CSI because he believed he was becoming "too comfortable" in the role, and wanted to pursue other opportunities. Because his role was so solidified in the long-running series, the job became monotonous, and he felt as though he wasn't growing as an actor.