Dr. Venkman turned from his screen and gazed quizzically at George. "That's funny. You don't look like a psychopomp." Typically a psychopomp - an entity that guides spirits from this plane of existence into another - looked like an animal, or something monstrous draped in robes. This one looked like an average girl, and furthermore she was hanging around on this plane when she didn't currently have any business. "Is that what they're wearing in Limbo this time of year?"
With this new knowledge, Venkman began to casually assemble a hypothesis in his mind. Maybe the serial killer operated outside of the usual laws of spiritual nature, meaning George wasn't notified of the murders in the usual manner. Then the bereaved spirits might be drawn to her, trying to pass on in order to balance the forces that had been disturbed. It didn't quite explain how they were returned to life, however. He guessed that the X-factor at work with the serial killer was likely at work in the resurrections, as well.