More than eight years later, Suffolk County, New York, police are fighting a judge's order to turn over a 22-minute 911 call made by the escort on the night of her disappearance.
Department lawyers argue that releasing the tape would jeopardize an ongoing investigation. John Ray, the attorney for Gilbert's estate, finds the police's refusal suspicious. He believes the police investigation has been compromised and the tapes, if they haven't already been destroyed, could reveal information damaging to the department.
"The public really has a right to know to what is on those tapes as well as Shannan's family," Ray said. "It will generate witnesses who are in the public who know what happened and perhaps will come forward."
The mystery over who might be responsible for the deaths of Gilbert and the other 10 people whose remains were discovered in a marshy area along a beach highway has stumped investigators for years. Police suspect a serial killer or killers used the area as a dumping ground, but no arrests have been made.
The Gilbert case stretches back to May 2010 when the 24-year-old from New Jersey vanished after making a 911 call from the home of her last client. In the call, Gilbert told the emergency dispatcher that someone was trying to kill her.
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Suffolk County Police launched a search, and in Dec. 2010, a K-9 unit found the first of 10 bodies buried near Gilgo and Oak beaches. The dead included eight women and a man wearing women's clothing — all suspected prostitutes, according to police — as well as a female infant.
It wasn't until December 2011 that investigators discovered Gilbert's remains in a remote marsh near Oak Beach, about a half-mile from where she was last seen.
A medical examiner's autopsy proved inconclusive. Then-Suffolk County Police Commissioner Richard Dormer said at the time that investigators believe she drowned or succumbed to the elements.
"She traveled at least half a mile, three-quarters of a mile, on foot through that muck," Dormer said at a news conference. "It would be very easy to get exhausted and fall down and not be able to move any further."
The lead detective in the case has argued in court filings that the department is still in the midst of an "open, active and ongoing" criminal investigation in the death of Gilbert. The detective, Patrick Portela, noted similarities in the sets of bodies that were found in the course of the search for Gilbert and that four of the dead women were escorts who, like Gilbert, used Craigslist and Backpage to advertise their services.
The disclosure of the recordings, Portela said in the court papers, would "compromise confidential information and interfere with and frustrate the Suffolk County Police Department's efforts" in the investigation.
"No arrests have yet been made in this investigation," Portela added. "As such, the [police department] has a critical interest in preserving the confidentiality of this active investigation, including its witness statements, which would necessarily include the calls made to 911."
The police were forced to make the case for withholding a recording of Gilbert's 911 call after her family filed a lawsuit against a Long Island doctor claiming he had opened his home to Gilbert but failed to keep her safe. The doctor has denied the allegations, and much of the lawsuit was dismissed due to the statute of limitations.
A judge has twice ordered Suffolk County police to turn over the tape of the call, as well as others made by neighbors around the time of Gilbert's disappearance. In an October ruling, Judge Sanford Berland said the police department failed to provide "the slightest intimation" of how or why giving Gilbert's family access to the tapes would "compromise any aspect of their protracted investigation."
Berland added: "No criminal prosecution is pending nor, so far as can be discerned from the police department's bare assertions, is it anticipated that a criminal prosecution will be brought within any time frame that the police department is currently willing, or able, to articulate."
The Suffolk County Police Department has appealed the ruling. It declined to comment when reached by NBC News.
Six years after Gilbert's disappearance, a new tragedy befell her family. Gilbert's younger sister, Sarra, fatally stabbed their mother, Mari Gilbert, in July 2016. Sarra was convicted of murder in April 2017 and sentenced to 25 years to life in prison.
In an interview, the lawyer representing Gilbert's estate said her surviving family members want to hear the 911 tape for reasons that go beyond the police investigation.
"The family has the right to hear the voice of their sister," Ray said.
Tom Winter is a New York-based correspondent covering crime, courts, terrorism, and financial fraud on the East Coast for the NBC News Investigative Unit.
Rich Schapiro is a reporter for the NBC News Investigative Unit.