Now authorities say they have reason to believe they have found her remains, and they say the suspect in the case is also dead.
Diaz disappeared from the Dane's Country Store where she worked in the small community of Danevang. It was November 24, 1990. Ever since, her family says, they've hoped one day they might find her alive. "That's not the outcome we were hoping for," said her sister Irma Diaz. Through tears she described the family's underlying frustration, "It was just not knowing all these years."
VIDEO: Original story. Aired 12/18/1990
The fifteen year old was working alone the night that she vanished. At the time, store owner Shelly Nelson told Eyewitness News, "Nothing was taken, no money, no beer, no cigarettes and her car, purse and jam box were still here."
Investigators say new information brought to them a few months ago by family of a man who died of cancer this summer led them to a remote location near the Wharton-Matagorda county line... just a few miles from the store.
VIDEO: Family's heartbreaking response
There, they found what they describe as a shallow grave. "We got lucky. We got lucky," said equusearch's tim miller who made the discovery. on the first dig he said found a bra. then, a ring. Its that ring which investigators say they've linked to Rosemary. It was a gift for her Quinceanera. "We knew at that moment it was a ring and we all cried," said Miller.
Investigators are collecting the remains for further analysis. Forensic scientists should be able to positively identify the remains, and investigators are hopeful they'll also be able to learn more about how she died.
There is a suspect in the case, but investigators say he is already deceased. The suspect died some time ago of an illness, but he had made partial confessions to various family members in his last years of life.
Investigators say no one person ever had the full story, and it wasn't until just a few months ago that people started talking -- helping reopen the cold case.
VIDEO: Former Sheriff who worked on case speaks out
"I spent 47 years in law enforcement and this case is very frustrating, it always bothered me. I was hoping before I'd die I'd find out what happened," said Former Sheriff Wharton Co. Rudy Machala. "The search for answers brought the small community together."
The name of the suspect is being withheld at this time to protect the identity of those who helped investigators make the discovery.