3x10 - Body of Proof

A schizophrenic teenage girl at a mental institution is found murdered. When Megan and Tommy interview a young patient named Darby, who seems extremely sane and lucid, she tells them that she saw the killer climb down from a heating vent in the ceiling, but most shocking of all -- he meant to kill her, not the actual victim. Could the killer be Dr. Malcolm, the ward director, who is obstructing their investigation.