Murder or accident: How did Shannan Gilbert die?

The search for a missing escort leads to the discovery of more bodies -- is there a serial killer on Long Island?

In December of 2010, Suffolk County Police Commissioner Richard Dormer downplayed it, but there was little doubt about what his department was confronting. Four bodies had been discovered. All were young women in their 20s. All were online escorts.

One by one, the bodies were identified, and with each name, came the story of a troubled life cut short.

MELISSA BARTHELEMY: Disappeared July 12, 2009

Lynn Barthelemy's 24-year-old daughter, Melissa, had been missing for a year-and-a-half when the bodies were discovered.

"My fiancé and I were actually watching...but they were televising when - where they found the bodies...And we just looked at each other and we started cryin,'" Lynn recalled. "We had a sinking feeling that it was her. And then the next day when Suffolk's County Police Department contacted us and said that they needed to come to Buffalo and speak to us, we just knew."

Photos: Crime scene and victims

After graduating from beauty school, Melissa moved from her small hometown near Buffalo to New York City to work as a hairdresser.

"I was terrified," Lynn said. "I mean, a big city like that. I'm like, 'they're gonna eat you alive.' But she was of age and all I could say was 'be careful' and call her all the time."

What Lynn didn't know at the time was that Melissa was actually working as an escort. In mid July 2009, after several days with no word from Melissa, Lynn panicked.

"Did she get hurt?" Lynn wondered. "We pull out the phone book, we get on the internet. We start callin' hospitals."

Lynn also contacted the NYPD to file a missing persons report, but she was in for a rude awakening.

"They didn't wanna hear anything," she said. "'She's 24. She's not on any psych meds. She's not missing. She's where she wants to be.' And that happened for three days consecutively."

Even the family's attorney, Steven Cohen, couldn't get police to take notice.

"I contacted them," he 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 disappeared, her 15-year-old sister, Amanda, got a call from Melissa's phone.

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

"And this voice is saying, 'Oh, this isn't Melissa,'" Cohen explained. "...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."

Suffolk County police told "48 Hours" that they believe the caller was, in fact, Melissa's killer.

And the calls didn't stop coming. Eight in total, including one in which the likely killer spoke to Lynn. That time, he claimed he was with the NYPD and wanted to know if she had filed a missing persons report. But mainly he focused on Melissa's little sister and called to terrify her.

"And the killer said some pretty horrible things to Amanda," Cohen told Moriarty. "Sexually explicit things as to what he had done to Melissa, sexually explicit things as to what he was going to do to Amanda."

After the third call, police asked Verizon to tap Amanda's phone.

Authorities were able to trace some of the calls to a handful of busy locations in midtown Manhattan; near the Port Authority, the Empire State Building and Times Square. The caller always hung up before he could be identified. But the police were learning a lot about him based on his voice.

"I believe that he is between - his late 20s and his late 30s," Cohen said. "I've been led to believe by Amanda and by the data that I have that he is a white male."

Much has been made about the caller's familiarity with police investigative techniques. He knew what locations would be difficult to trace and how long he could stay on the phone.

"What about talk that this killer might even be part of law enforcement? Do you believe that?" Moriarty asked Retired New York City homicide Commander Vernon Geberth, who has analyzed more than 300 serial murders in the United States.

"No," he replied. "Anybody who watches television - you're bombarded by forensic programs - you can't turn on the TV anymore without 'CSI,' 'NCIS,' 'Law and Order,' and 'Law and Order: Special Victims, and '48 Hours,' OK? So people are watching and they're learning."

"Do you believe this person could be watching the interview right now and enjoying the attention?"

"Well, yes, because he's not identified. In his position right now, he's in a position of superiority," Geberth replied. "He has beaten the police; no one knows who he is. He's immune. It's almost like feeling invulnerable."

That sense of confidence may have driven the caller to reach out to Amanda one last time on Aug. 26, 2009, with a chilling message.

"And he said, 'Do you know what I did to your sister?' And she said, 'No.' He said, 'Well, I killed Melissa,'" Cohen said. "And he also left one of the conversations with the threat that he knows where Amanda lives and might come after her."

"What kind of person calls the little sister of a victim and says, you know, 'You think you're gonna see your sister again? 'Cause I just killed her,' Moriarty asked Geberth.

"A psychopathic, sexual sadist. They're dangerous, a human predator," he replied.

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

Asked if he believes this man will probably kill again, Cohen said, "Oh, there's no question...he is getting better and better at it. He is very controlled, very calculating in what he does. And he's on a mission."