UPDATE Sept. 12, 2017: A carpenter convicted of killing two prostitutes in the 1990s may be responsible for at least one of the 10 unsolved killings of people along a Long Island beach highway.

It may be the most infamous unsolved serial killer case on the books today, a murder mystery Crime Watch Daily has been following closely near Gilgo Beach, Long Island, where at least 10 bodies, mostly strangled young women, were found hidden in the brush. Is convicted murderer John Bittrolff connected to these killings? WPIX's Mary Murphy investigates.

Dec. 11, 2015:

It is one of the most baffling serial murder cases in modern American history, centered on a lonely stretch of sand on the south shore of Long Island, New York.

Gilgo Beach is a small community with a big mystery on its hands. It's where the bodies of 17 brutally murdered young woman have been found, many of them dumped in the surrounding dunes over a period of 20 years.

Not a day goes by that Mari Gilbert doesn't long for an answer in the death of her 22-year-old daughter Shannan.

Shannan had traveled from New Jersey to Oak Beach, Long Island, near Gilgo Beach, for a date. Later that night she was seen running through the neighborhood in a blind panic.

Gilbert family attorney John Ray has logged more than 700 hours on the case tracing her steps that night.

"She runs out of his home, she calls 911, she runs through the neighborhood knocking on doors, and whenever someone comes to the door who attempts to call the police but not take her, in she runs," said Ray.

And on May 1, 2010, that's the last anyone's heard of her.

Police cleared Shannan's date of any foul play. But she remained missing, and her family pressed for answers.

Former Commander of the NYPD Bronx Homicide Task Force Vernon Geberth has worked hundreds of murder cases and has studied this one closely.

"I don't think if Shannan Gilbert's mother didn't make the hue and cry she did, they would've done anything in Suffolk County," said Geberth. "If that was some millionaire debutante from the Hamptons I'll bet there would have been a whole crew looking for her."

Seven months later police search teams find the body of a young woman in the weeds off Gilgo Beach. But it's not Shannan.

Police widen the search area, still looking for Shannan.

Instead they find the remains of three more young women, all of them like the first discovered body: strangled, stuffed in burlap bags and hidden in the scrub off the beach.

Detective Dominick Varrone is a former Suffolk County chief of detectives who worked the case.

"The initial four bodies, we believe, were simply the killer simply drove along Ocean Parkway pulled off to the side of the road, the shoulder we believe removed the body and tossed it into the bramble as far as he could," said Varrone.

As the case progressed, Mari Gilbert became friends with Lorraine Ela. Ela's daughter Megan Waterman was among the first victims found, six months after she went missing.

The following spring police found six more bodies near Gilgo Beach.

The area along Gilgo Beach where the bodies were found is cold, quiet and very dark in the winter, but in the summer, at the time of the discoveries, there were a lot of people here.

Out of those 10 bodies, not including Shannan's, only five have been positively identified.

When added with remains found in the wider area over two decades, some believe the killer may be responsible for up to 17 murders, more than half them unidentified. The nameless victims include an Asian male in women's clothing, and a mother and her young child.

"It was a unique case because there was a disposal site," said Gebreth. "Not many serial killers maintain a body disposal site. Now, the psychological significance of a disposal site is that the killer goes back to this site much more comfortable second, third, fourth time, but gets to relive the event, because the total control of another being is what drives them."

Police got their first break in the case when they made a connection with 24-year-old victim Melissa Barthelemy. Someone, possibly her killer, used her cellphone from Manhattan to call her sister with a chilling message.

"This guy's a sexual sadist, no doubt about it," said Gebreth. "For him to take the phone and call Melissa's little sister and vividly describe what he did to her sister and how he's going to do it to her, that is him reliving the crime. The problem was the calls were made from midtown Manhattan and they couldn't find it, too many calls. Then he disposed of the phone."

The disappearance of 23-year-old Shannan Gilbert leads to the discovery of multiple bodies of young women, most of them strangled and tossed into the dunes around Gilgo Beach.

Former Bronx Homicide Task Force Commander Vernon Geberth says the full-court press should have started the day police found the first four bodies wrapped in burlap.

"Everybody knows somebody in Long Island, and I'm telling you for the record that the person who did it was highly stressed and people close to him would've seen a sudden change in his demeanor, he wouldnt've gone to work, he would have started drinking, he would have acted strange and he would have left the area," said Geberth.

But some, like the Suffolk County Police, think that there may be more than one killer involved. It's not the first time the experts are divided.

"The D.A. says there are probably two killers because of what they found," said attorney John Ray. "One where two bodies were chopped up, the others where bodies were intact. So they say that can't be the same method, I disagree. The killer can easily learn from his own experience on how to kill and what to do about it and he changes over the course of years. And if the killer is intelligent, which this killer clearly is, they do change their methods."

Then a major break in the case: 19 months after she went missing, the decomposed body of Shannan Gilbert is found near Oak Beach, Long Island. It's the same place she made her last fateful 911 call a few miles down the road from Gilgo Beach.

The search for Shannan's body is the linchpin that uncovered a virtual mass grave of young victims. But now the Suffolk County Police speculate that Shannan was not murdered, but accidently drowned herself in a drug-induced haze.

That idea that is soundly rejected by Gilbert family attorney John Ray.

"The theory that Shannan used cocaine, became confused, tripped, fell, drowned and somehow her jeans were pulled off by the bramble bushes is the theory of the Suffolk City P.D. It's an absurd theory," said Ray. "The water in the marsh is only inches deep. She would have to work at it to drown and to end up where she did face-up makes no sense whatsoever."

The Suffolk County Police have refused to comment on the case to Crime Watch Daily.

But John Ray called for a second autopsy on Shannan's body by high-profile forensic pathologist Dr. Michael Baden. And accidental drowning is nowhere in his conclusions.

"The bone in her neck, known as the hyoid bone -- it's a shaped bone, small, maybe about that big, had two ends or 'horns' upon it," said Ray. "The horns are missing from the hyoid bone of Shannan Gilbert. That indicates strangulation. Virtually every girl who's strangled, virtually every girl's hyoid bone breaks."

At this point, every stage of the investigation has been one more step into the unknown. But things get even more strange with the introduction of Dr. Peter Hackett. He's a resident of Oak Beach, where Shannan was last seen. And he claims he saw her on the last night of her life.

Only two days after Shannan Gilbert's disappearance, her mother Mari hears from a mysterious stranger. He's the first to alert her that something may have happened to her daughter.

"I got a phone call from a man, he said 'My name is Dr. Peter Hackett, I'm calling to see if your daughter Shannan is there,' and I said 'No, why? Who are you?'," said Mari. "And he said he ran a wayward house for girls, Shannan was the, she was upset, she needed to get off the streets. He'd given her some medication to calm her down, and he was worried about her, is she home?"

Dr. Peter Hackett is a former physician for the Suffolk County Police who was living in Oak Beach when Shannan went missing in the neighborhood.

"I had asked him, 'How did you get my number?'," said Mari. "And he told me that it's policy that if anybody goes to his house that he has to have contact information, and I knew that was wrong. Shannan, none of my children would ever give my phone number to anyone."

Gilbert family attorney John Ray says Dr. Hackett is a man who deserves a close look by investigators.

"He never ran such a home. It's a complete fantasy, a drug-infused fantasy, perhaps, but fantasy," said Ray. "And why is he making that particular fantasy in this case, why has that not been thoroughly investigated?"

At first Hackett denied making any phone calls to Mari Gilbert

"Dr. Hackett denied on national television and in writing that he ever made those phone calls," said Ray.

He later admitted making the calls when the phone records were made public. But Hackett claims he never said anything about running a home for wayward girls, and that he never met Shannan Gilbert."

Hackett said he had no knowledge of Shannan Gilbert, never saw her, never met her, never talked to her, that this was all nonsense.

Crime Watch Daily reached out to Dr. Hackett's attorney James O'Rourke several times, with no response. In an interview with Newsday, O'Rourke said of his client: "He never saw her. He never met her. He never treated her."

And police have not identified Hackett a suspect.

When Shannan's body is finally found 19 months after she disappeared, her mother, Mari Gilbert, gets the shock of her life when police take her to view the scene.

"One of the detectives picked me up in his car, drove me, took me to Peter Hackett's house, took me on his back balcony of his house and from his balcony they pointed and showed me where they found my daughter behind his house," said Mari. "I'll never forget that day."

The case has heated up with the Gilbert family suing Hackett for the wrongful death of Shannan.

Dr. Peter Hackett has given three depositions in regards to the Shannan Gilbert civil suit filed against him. He claims he had nothing to do with her death.

Peter Hackett has refused a formal interview with Crime Watch Daily. But we waited for him in the courthouse parking lot after his deposition to see if he had anything to say. He denied running a home for wayward girls.

But Shannan's mother Mari says he absolutely told her he ran a home for wayward girls in his phone call to her.

Asked if he had anything to do with Shannan Gilbert's death? "Of course not," said Hackett.

There is some new hope for the families of the victims at Gilgo Beach. The commissioner and chief of the Suffolk County Police have been replaced. The new team is now joining forces with other law enforcement agencies across the nation, something the previous administration declined to do.