The remains of Harold Dean Clouse, 21, and Tina Gail Clouse, 17, were discovered by a dog in an area near Houston in January 1981.
Mr Clouse was found beaten, bound and gagged, while his young wife had been strangled.
The couple had left their home in Florida with their one-year-old baby, Hollie Marie Clouse, so that he could work as a carpenter in Texas.
After they disappeared their car was returned to their family, who believed they had joined a religious group and no longer wanted to be contacted.
After the bodies were found, the Harris County forensic artist put out a reconstruction drawing but no one recognised the victims and the case went cold.
In July 2011 the Harris County Medical Examiner’s office exhumed the bodies to extract DNA to see if the pair were related and last year, media company Audiochuck, which produces true crime podcasts, founded new genealogy research.
Forensic anthropologist Deborrah Pinto of the Harris County Institute of Forensic Sciences then worked with Identifiers International, and together they quickly identified Harold Clouse as one of the victims, followed by Tina.
“I’m very thankful to have been given the opportunity to work on this case. It was a difficult case emotionally,” said Identifinders senior forensic genealogist Misty Gillis.
Genealogist Allison Peacock, who also worked on the case, called the family to let them know of the breakthrough, and was immediately asked about the daughter that investigators had previously not known about.
“I will never forget my son and his Tina, I went years in pain wondering where they were. Now I can only keep praying God lets me see my granddaughter one day,” Donna Casasanta, Harold’s mother, told USA TODAY.
But despite solving one mystery, another remains: whatever happened to baby Hollie Marie?
No baby was found with the couple’s bodies and no Baby Doe cases have been found that match the youngster’s description.
“I spent years waiting to get a call from my son or calling police stations each time a new male body was found. I spent years with my chest on fire, just waiting,” Ms Casasanta told USA TODAY.
Identifiers International says that it is working to have Hollie Marie Clouse, who would now be 41, listed with the National Center for Missing and Exploited Children.