Leelee Sobieski's character-Lauren Douglas (really Lydia Dougherty) is the murderer.
Lydia Dougherty is a student lawyer enrolled in Dr. Jack Graham's (Al Pacino) Forensic Psych class as "Lauren Douglas" (same L. D. initials) to 'watch him squirm' as she sends automated messages to his cell phone ("You have 88 minutes to live. Tick, tock, doc").
She is manipulated by Jon Forster, the guy on death row because of Dr. Graham's testimony, to emulate Forster's m.o. in the slaying of more victims, thus casting doubt on Graham's expert testimony in Forrester's trial & subsequent conviction.
The movie is pretty good, although I've seen better from Al Pacino.
Before the movie is over, you suspect eveyone!