It was her disappearance that led to the discovery of 10 bodies dumped along Ocean Parkway in Suffolk County.
It took Suffolk County authorities a year and a half to find the body of Gilbert and "48 Hours" correspondent Erin Moriarty told CBS 2 that the investigation was hampered from the very beginning.
Sherre Gilbert said the Suffolk County police failed her sister when Shannan Gilbert went missing from the gated community of Oak Beach in May 2010.
"I don't feel that the police took the investigation seriously," she said. "It just didn't seem like they were doing anything to really try to search for her."
New details are only now emerging about the missteps and miscommunications early in the investigation, beginning with Shannan Gilbert's 23-minute call for help shortly before she disappeared.
Former Suffolk County Chief of Detectives Dominick Varrone has released new details about that disturbing 911 call.
"She's saying, there's someone after me; there's someone after me," Varrone said. "It's a girl who clearly believes she's in harm's way."
Moriarty: "Does she say who she's afraid of?"
Varrone: "She just says they're trying to kill me."
Since Shannan couldn't tell the operator where she was, that alarming call that should have been handled by Suffolk County was immediately transferred to the New York State Police, Moriarty reported.
Moriarty: "Are you saying that basically Suffolk County never heard that 911 call that morning?"
Varrone: "Absolutely correct."
That diversion cost investigators precious time as Shannan, who was still alive, raced through the neighborhood, Moriarty reported.
Gus Colletti and another resident of Oak Beach also called 911.
"I was in the bathroom shaving. All of a sudden, I heard screaming out here and banging on that door," Colletti said. "Yelling, 'Help me, help me, help me.'"
Officers responded, but didn't conduct the search they would have if they had known about Shannan's frantic call for help. Instead, they left.
The "48 Hours" investigation also reveals another reason for a delay.
Gilbert's family was required to file the missing person report in Jersey City, N.J., where she lived, instead of where she disappeared.
Suffolk County police didn't get the report, which contained vital information, until much later, Moriarty reported.
In the report, the family told investigators in New Jersey that there was surveillance tape where she disappeared and it was not conveyed to Suffolk County authorities until later when the tape was already taped over.
The show airs Saturday night at 10 p.m. on CBS.
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