Suffolk County Police Commissioner Geraldine Hart revealed a photograph of a black leather belt embossed with the letters "WH" or "HM," depending on how it's held.
Hart said at a press conference she believed the suspect in the murders "handled" the belt, but would not elaborate.
Police said they suspect a serial killer is responsible for the murders.
In 2010 and 2011, the remains of 10 people were discovered in weedy sections of Ocean Parkway near Jones Beach.
Six women were identified, but the remains of the other four people, including a toddler, haven't been. Half of the identified victims worked as prostitutes, police said at the time.
Authorities were searching for Shannan Gilbert, a sex worker from New Jersey, when they discovered the remains. Gilbert is not believed to have been tied to the other deaths because she "doesn't match the pattern of the Gilgo Beach homicides," Hart said.
Hart urged the public to come forward with any information involving the murders, including on the belt, which was collected at the initial crime scene. She would not say the size of the belt, where exactly it was found at the crime scene and declined to say whether DNA had been found on it.
The photo of the belt will be the first piece of evidence featured on a new website, gilgonews.com, dedicated to providing updates in the case and collecting public tips.
When asked why she was releasing the image now after having had it for all these years, Hart said, "Now is the time to release this information," without providing details.
Hart also announced Suffolk County had been granted approval to apply to the case a forensic technique not yet authorized in New York.
DNA samples from the unidentified victims have been sent to the FBI for genealogical testing. Hart said she hopes the testing will be able to identify possible relatives and, in turn, help identify the remaining victims.
"We remain steadfast in our commitment to deliver justice," she said at the press conference, which was held at Suffolk Police Headquarters in Yaphank, New York.
While officers were searching for Gilbert, on Dec. 11, 2010, they came across the remains later identified as Melissa Barthelemy. Two days later, during another search, authorities found the bodies of three more victims: Amber Lynn Costello, Megan Waterman and Maureen Brainard-Barnes.
In March 2011, the remains of Jessica Taylor were located nearly eight years after her partial remains were found in Manorville, New York .
Three more bodies, an unidentified woman, a 2-year-old girl and an unidentified Asian male believed to be between 17 and 23 , were found April 4, 2011.
A week later, the last two of the 10 victims were found in Nassau, including the toddler's mom.
Gilbert's remains were found in December 2011 in Oak Beach.