Murder by Numbers

Police are puzzled by a pair of murders linked by a mysterious note, not realizing the killer has a target in mind.

In Billings, Mont., firefighters respond to a fire at a duplex and find two bound and gagged bodies; the victims' house mate is nowhere to be found, prompting police to question whether they're searching for the killer or another victim.

A woman is found murdered on the Appalachian Trail and her husband is missing, leaving authorities to wonder if he's the killer or another victim, then more bodies begin to appear.

Firefighters find the bullet-ridden bodies of Robert and Dorothy Pape while responding to a call about smoke billowing from a garage, in which they find a truck on fire; later, police realize the smoldering truck does not belong to the homeowners.

When a woman is found brutally murdered in her home, all signs point to a local troublemaker, then more bodies start piling up, making police realize they may be dealing with a serial killer, who is preying on the town's most vulnerable citizens.

In Amherst, N.Y., a missing college student's body is found hidden in the woods near a bike path, making the townspeople realize that the predator who has been stalking women for years has escalated from rapist to killer.