LI killer probe: "48 Hours" has new details

"48 Hours Mystery" reveals new, exclusive details tonight about the investigation of a suspected serial killer on New York's Long Island. "48 Hours" Correspondent Erin Moriarty shared a preview of the episode on "The Early Show."

In the early morning hours of May 1, 2010 Shannan Gilbert, a 23-year-old escort, disappeared in the dark shadows of a gated community in Oak Beach, Long Island. Shannan's sister, Sherre, says Shannan made a panicked call to 911 and was seen running along the roads, Moriarty reported.

"48 Hours: Mystery": The Long Island Serial Killer

"She was in fear of her life that night," Sherre told CBS News. "And that was it. She just disappeared into the night."

Six months later, the search for Shannan lead to a gruesome discovery: four female victims, their burlap-wrapped bodies dumped on a secluded stretch of beach on Long Island.

Police Commissioner Richard Dormer said following the discovery, "We could have a serial killer."

One-by-one, the bodies were identified. Shannan Gilbert was not one of them. But, like Shannan, all the victims worked as online escorts, and they all went missing while going to meet clients.

Lynn Barthelemy's 24-year-old daughter, Melissa, was last seen in New York City, more than a year-and-a-half earlier.

Lynn Barthelemy said, "My fiance and I were actually watching televising when - where they found the bodies. And we just looked at each other and we started crying. We just - we had a sinking feeling that it was her."

Now, it is one of the largest criminal investigations in Long Island's history. But when Melissa Barthelemy vanished, even the family's attorney, Steven Cohen, could not get the New York City Police Department to take notice.

"I contacted them," Cohen said. "And they said, 'She's a hooker. She's a prostitute. She was - she's an escort. We're not going to assign a detective to this."

About a week after Melissa Barthelemy disappeared, her 15-year-old sister, Amanda, got a call from Melissa's phone.

Lynn Barthelemy said, "When Amanda answered the phone, you know, she was so excited. 'Oh my God, Melissa's finally calling me.' And then, there's a guy on the other end."

Cohen added, "And this voice is saying, 'Oh, this isn't Melissa.' He was soft-spoken and had a very controlled and comfortable manner of speech, which made his horrific messages all the more devastating. And he began to toy with her. And for the very first time, she heard the voice of the killer."

Lynn Barthelemy said of the caller, "All I can say is, he's sick. And he's gonna make a mistake. And we're gonna catch him."

Moriarty discussed the case on "The Early Show." Click on the video for her insights into the case.

You can catch "48 Hours: Mystery: Long Island Serial Killer" tonight on CBS at 10/9c. Moriarty will be on Facebook during and after the "48 Hours Mystery" show, along with family members of two victims. To join in, go to the "48 Hours Mystery" Facebook page.