Hunting Humans: Serial Killer Stalks Homes Along Highways

Darlene Ewalt of Harrisburg, Pa., was sitting on her patio at 2 a.m., talking on the phone to a friend about an upcoming cruise. She did not know it, but on that hot summer night of July 13, 2007, she was being watched, hunted.

Then, out of the darkness, came an assailant wielding a long knife.

As Darlene's husband slept upstairs, the killer slit Darlene Ewalt's throat, and stabbed her so violently she died in minutes.

"She screamed, 'Oh my God, Oh my God,'" her husband Todd said.

Darlene's family was devastated.

"I don't go outside alone anymore at night," her 21-year-old daughter Nicole Ewalt said.

Nick Ewalt, 24, said he is also haunted by the tragic memory of his mother's death.

"A lot of nights I don't go to bed, I don't sleep," he said. "I just lie in bed thinking about all that happened."

Darlene Ewalt's husband, Todd, said the murder of his wife "devastated my life. I've never felt anything close to that. I felt like someone ripped my heart out of my chest."

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This was no ordinary homicide. The FBI suspects it was done by a serial killer, a truck driver using the highways to randomly target victims, to hunt them, kill them and then escape.

The agency believes the man charged with murdering Darlene Ewalt is just one of hundreds of killers who may have gone on similar killing sprees undetected along the nation's interstates.

Police believe Darlene Ewalt's killer calmly walked back to his rig parked at a nearby truck stop and drove away, leaving the crime in his rearview mirror.

Evil Comes to Bloomsbury

Four days after Ewalt was killed, a York, Pa., woman narrowly escaped a shadowy figure slashing at her neck as she was startled out of sleep on her couch.

But 11 days later, on July 28, 2007, in Bloomsbury, N.J., 38-year-old Monica Massaro would not be so lucky.

Around midnight, the killer pulled into a truck stop near her home. He hid in the shadows, moving silently down the picturesque streets in the middle of the night, looking for unlocked doors.

Detective Sgt. Geoffrey Noble of the New Jersey State Police told "Nightline" what happened at the scene of the crime.

"He walks up these front steps and this front door, the door was unlocked, and he walked right in through her front door," Noble said. "He went through her house. He spent time in her house. At some point he ends up in her bedroom on the first floor."