CBS

Season 13: Episode 17 - Death Of The Class

Death Of The Class: David Phillips begrudgingly returns to his old high school for a reunion, only to find the former prom queen, Becca Sloan, dead from an apparent stab wound to the neck in a storage room. The CSIs are called in to investigate, and Phillips gives them a rundown on all of his old classmates as they begin to process the evidence. Upon finding a piece of glass from a copy machine embedded in Becca's palm, the spotlight falls on Kelly Nash, one of Becca's best friends in high school. Kelly, however, has an alibi, and the CSIs are left to dig through scarce evidence to find the real killer. They catch a break when Morgan uncovers a trace of bloody tan paint, which came from a ceiling panel in the storage room that was accessed very recently. When Finlay goes to examine the panel, she discovers a pipe bomb in the compartment. The CSIs then begin to suspect Sean McHenry, Becca's high school boyfriend, of killing her and planting a bomb to get revenge on the school board that fired him. However, upon closer analysis of the bomb materials, it's clear that the bomb is very old, and was likely planted way back in 1998. David Phillips eventually concludes that the bomb was planted by Max Desario, who wanted to get revenge on the kids that drove his friend, Caroline Hartwell, to commit suicide. Desario planted the bomb, but never set it off, and he saw the reunion as his chance to safely disarm the device without anyone getting hurt or knowing about it. Just as he was about to apply liquid nitrogen to the bomb to diffuse it, Becca walked in on him and he accidentally splashed her with the liquid nitrogen, which she swallowed. The nitrogen blocked her airway, so Max tried to perform an emergency tracheotomy and failed, then fled the scene. Max admits to killing Becca, and the CSIs arrest him, along with former valedictorian Janet Campbell, who tried to hide the evidence for him.