"When Amanda answered the phone, you know, she was so excited," Barthelemy 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 ... 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.

"I was just hysterical, totally hysterical," said Barthelemy.

The calls didn't stop coming. There were eight in total, including one in which the likely killer spoke to Lynn. 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. 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."