Gilbert's remains were discovered in a marshy area in Oak Beach, Long Island, nearly 20 months after she vanished, police said. Oak Beach is about 9 miles from where 10 other sets of human remains
were found, leading authorities to a hunt for a possible serial killer.
Her family has long maintained that she died at the hands of a serial killer, but authorities have offered conflicting theories about the death, including a drowning and drug overdose.
At a news conference, John Ray, an attorney for Gilbert's family, said the independent autopsy's "very disturbing conclusions" should change the focus of the investigation into the deaths.
"It's time now ... for the police department to wise up," Ray said. "It's time now for the police department to do the job that they have failed to do up until now and to investigate the death of Shannan Gilbert as a murder."
Holding a photo, Mari Gilbert told reporters, "This is my daughter Shannan. She was not perfect. No one is perfect. But she was loved. She was cared for. She does not deserve to be forgotten."
In a statement, Suffolk County Police Commissioner Timothy Sini said his department is "doing everything it can to solve the Gilgo Beach homicides and that is why the Department recently partnered with the FBI."
Suffolk County Police announced in December that the FBI had joined the Gilgo Beach investigation
Later Friday, Sini said homicide investigators had reviewed a letter highlighting the new autopsy findings.
"It provides no additional information, and concludes, as did the Suffolk County Office of the Medical Examiner, that there is insufficient information to determine a definite cause of death," Sini said.
Though Gilbert's death certificate listed the cause and manner of death as "undetermined," an independent autopsy by former New York chief medical examiner Michael Baden concluded "there is no evidence that she died of a natural disease, of a drug overdose or of drowning."
"There is insufficient information to determine a definite cause of death, but the autopsy findings are consistent with homicidal strangulation," Baden wrote in the report released Thursday.
Baden wrote that nearly all of Gilbert's recovered skeletal remains appeared normal but the "larynx was missing and only the body of the hyoid bone was found; the two greater horns of that neck bone were missing. These structures, the larynx and the hyoid bone, are often fractured during homicidal manual strangulation."
Police have said many of the identified victims, including Gilbert, had advertised prostitution services on websites such as Craigslist.
Gilbert, 24, vanished in May 2010
after visiting a client, according to police.
The search for Gilbert led police to four bodies stuffed in bushes along a quarter-mile stretch of Ocean Parkway in Oak Beach. All four were later identified as women who had advertised online for prostitution services.
The bodies were found in various stages of decomposition.
Ray said Baden's findings are "consistent with the fact that Shannan was strangled to death, just like the other four who were found along Ocean Parkway in Oak Beach, who were clearly strangled as well."
Additional remains were later uncovered in neighboring Gilgo Beach and in Nassau County, about 40 miles east of New York City.