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."
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.
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."
According to the police investigation, Massaro was sleeping in her bed when the killer came into her bedroom, but while he was there she woke up. She turned on her light and he attacked her with a knife, police said.
"The first significant wound was a cut to her throat, a fatal wound," Noble said. "The severity of the wound was such that she bled out and died very quickly. After her death, he continued to stab her multiple times."
The killer returned to his truck and left town, not leaving a trace of evidence linking him to the scene, Noble said.
"She died alone in the hands of this monster," he said. "She didn't have time to call for help or run. She was cornered. She had nowhere to go."
Less than 24 hours later, more horror began to unfold in Chelmsford, Mass.
At around 11 p.m. on July 29, 2007, a woman in an apartment complex there saw a man hiding in the bushes and called police. When officers arrived, the man had disappeared.