Episode 24 of "Numb3rs"
s02e11 Scorched

An arsonist sets a fire at a car dealership that kills a sales person. The name of an extremist environmental group is spray-painted on the scene, but the group denies involvement. It becomes Don's task to determine whether the group or someone else is responsible. Charlie is called in to help figure out if there is a pattern to the fires that would help provide a profile of the arsonist.