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…

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…

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.

A monster hunts the streets of Baton Rouge, La., raping and murdering three women; when one brave woman survives a vicious attack, evidence leads police to the hunter.

In Bellevue, Wash., a twisted murderer preys on young women leaving their bodies in sadistic poses; as police work to draw crucial connections, an unconventional clue surfaces and leads them to an unlikely killer hiding in plain sight.

In Alexandria, Va., a sheriff's wife answers the door to a blaze of gunfire, causing the community to buzz with theories; then, more citizens are shot at their front doors, leaving everyone wondering who could be next.

In Orange County, Calif., four men are stabbed by a mystery man in a month's time; no one makes a connection to the gruesome double murder of a woman and her son two months earlier except the man sitting in prison for killing his own…

Dallas has its dark side, but nothing compares the horror of a series of women found murdered with their eyeballs surgically removed; a pair of beat cops use their street savvy to lead detectives to a sadistic killer and his bizarre motive.

A woman's husband is murdered by an intruder wearing a clown mask. After three more victims are killed by a deranged clown, police realize they're dealing with a serial killer.

The Necktie Killer strikes fear in the hearts of Atlanta's citizens; detectives from neighboring counties piece together correlative clues and distinctive apparel left behind at each crime scene.

A series of murders in California bear eerie similarities, but police have trouble connecting them -- that is until unique ballistics, mysterious surveillance, and a bloody cell phone lead them to three criminals on a deadly revenge spree.