For nearly a decade, the gruesome discovery of 11 bodies over two years near a beach on Long Island has terrified residents and stumped investigators.

Eventually, the police linked four of the bodies to one serial killer, whom they could not find. Years passed, and the case remained a mystery.

Then, on Thursday, investigators at a news conference showed a photograph of what they said was a significant piece of evidence collected “at the initial stage” of the investigation: a black leather belt, embossed with a pair of letters about half an inch tall.

Investigators believe it spells out the initials “W H” or “H M.”

“We do believe that this item was handled by the suspect,” Geraldine Hart, the Suffolk County police commissioner, said at the news conference at Suffolk County Police Department headquarters. She declined to say whether the belt was used as a murder weapon.