Kyle Clinkscales’ parents never gave up looking for their son after he vanished in Georgia in 1976. They became crusaders on behalf of missing persons.

Every time Louise and John Clinkscales left their home in LaGrange, Georgia to look for their son—who had been missing for decades—one of them would leave behind a note.

Kyle’s parents wanted their only child to know that a lot had changed since he was last seen in 1976 at the age of 22 in case he returned while they were away. The Clinkscales would write that they had a spare car key for him on the dining room table because they loved him.

They looked everywhere, said Ms. Clinkscales’ sister, Martha Morrison, 88, of Oxford, Alabama, in an interview.

Before the authorities made a remarkable discovery in December 2021 involving a 1974 Ford Pinto sticking out of a creek, Mr. Clinkscales’ identification inside the rusty car, and about 50 skeletal fragments encased in the mud, Mr. Clinkscales’ parents passed away.

The remains of Mr. Clinkscales were identified last week by the Georgia Bureau of Investigation. Given the age of the remains, Erin Hackley, the coroner in Troup County, Georgia, warned that it might take months for investigators to identify a cause of death, if they can do so at all.

Residents of LaGrange, which is about 70 miles southwest of Atlanta, have been enquiring about a sports-obsessed young man who was attending Auburn University in Alabama for a little over 47 years.

He was figuring out what career to pursue and starting to look for his place in the world at the university. Mr. Clinkscales, a sophomore at Auburn University, left his part-time job at a bar in LaGrange on January 27, 1976, and began the roughly 45-minute drive there.

Investigators believe that something happened at some point in his trip, and his whereabouts remained a mystery ever since.

A driver in Cusseta, Alabama, about 30 miles southwest of LaGrange, was on a two-lane road on December 7, 2021, when he noticed the hatchback of a rusted vehicle sticking out of the creek and called the authorities, according to Sgt. Stewart Smith of the Troup County Sheriff’s Office. What, after all this time, had made the car visible from the road was unclear.

The fact that the road was probably not Mr. Clinkscales’ primary route to Auburn suggests that the creek in Chambers County, Alabama, outside of LaGrange, was never searched. However, it’s possible that it was an alternate route.

Just 11 months after Ms. Clinkscales passed away in January 2021 at the age of 92, Sergeant Smith recalled how deputies felt when they realized the oldest missing person cold case in the county was finally coming to an end.

In those first few weeks after he went missing, the Clinkscales and the Troup County Sheriff’s Office had conducted a thorough search for Mr. Clinkscales. Lakes had been drained. Reward promises were made. Officers combed the woods for any hints.

The search was a passionate, all-consuming endeavor for Louise and John Clinkscales, mirroring dozens of other missing-person cases across the nation, with family members pleading for information, searchers getting weary with each fruitless effort, and members of a worn-out community looking on, aghast that something so haunting could have happened to one of their own.

Their determination was the source of admiration for many.

Because Kyle Clinkscales had always enjoyed New Orleans, the couple purchased advertisements there pleading for assistance in finding their son. According to an interview they gave to The Auburn Plainsman in 1978, his parents sent letters to all of the state’s police departments because he had loved Hawaii when he had visited there once on vacation.

And when information about a person matching his description—strong jaw, shaggy brown hair, thick eyebrows—was received, they traveled to the locations where it was reported. The Clinkscales had handed out nearly 5,000 bumper stickers in search of answers two years after their son vanished.

They became supporters for families of others who had missing relatives and tried to call attention to cases not as well publicized. The Clinkscales were among those invited to the White House in 1985 to meet with President Ronald Reagan about ways to address the issue of missing and exploited children