The last time Bernard "Bunny" Ross jnr's parents saw him was a Thursday morning in May 1977, when, they said, he left home in anguish.
It was suspected that the teen had swiped his aunt's pick-up truck - which was later found empty and intact on a dirt road in Maine, according to news reports from that time.
But Ross, then 18, was never seen again.
Now, nearly 40 years later, his parents have received an anonymous letter claiming some level of knowledge of Ross's disappearance, police told the Portland Press Herald.
"It's very upsetting to us," his mother, Carol Ross, 78, told the Washington Post today. "It's very traumatic, even given the length of time."
She said it was too painful to speak further about the mystery of her son's disappearance and said authorities have advised the family not to discuss the letter during the police investigation.
Police reports and missing children's posters from that time say Ross, known as "Bun" or "Bunny," disappeared from the home he shared with his parents in Fort Kent, a town in the northern tip of Maine.
The teen was spotted in a wooded area in Ashland, according to the Charley Project. He was clad in a chamois shirt, green vest and corduroy pants - and was on an anti-psychotic drug at the time, according to the organisation.
His parents told the Bangor Daily News that the community was "shocked".
"People had a hard time because there were no answers," Carol Ross told the newspaper. "It's not like there was a death. It was the unknown. . . . There was always the hope that he'd walk through the door one day."
Today, Bunny Ross would be 57.
After years of searching, his parents - Carol Ross and 80-year-old Bernard Ross snr - have never learned their son's fate.
"I go from thinking he's out there somewhere - maybe in a hospital or carrying on a new life," Carol Ross told the Portland Press Herald years ago. "Other times, I think he must be gone, because he would have called us."
But, she said, perhaps the most heart-wrenching moments were when authorities would call the couple to ask them to identify a body. They were terrified that their son had been found dead - and then upset that his fate was still unknown.
"We've had several calls," she recently told the Portland Press Herald. "You'd get your hopes up, but of course it would turn out to be someone else."
Years ago, the National Centre for Missing & Exploited Children created an age-progression image to show how Bunny Ross might have changed since his disappearance.
Then, earlier this year, Ross's parents received a letter mentioning a missing persons story that appeared in the Kennebec Journal that referenced Ross's case; the letter proposed that the newspaper write another story about him, according to the Portland Press Herald.
Maine State Police Lieutenant Troy Gardner told the Press Herald that he contacted the media in the hope that more stories might encourage the anonymous author to come forward - though he declined to share detailed information about the letter's contents with the media.
"If there was details in the letter that we felt were important to the investigation, we wouldn't release them," he told the Portland Press Herald. "That's common sense. It protects the integrity of the investigative process."
Still, Gardner said: "I've never had anything like this happen in my career. Basically, all we're doing is extending an olive branch, saying we want to make contact with this person. Of course, there's no way of knowing whether the letter's truthful or the information is accurate, but we are asking for whoever wrote the letter to please contact us."
It might be a hoax, Gardner said, but police are still investigating.
"Obviously, if it is a hoax, that's a horrible thing to do to somebody who's been missing their son since 1977," he told the Bangor Daily News.
Gardner told the Bangor Daily News that there's always been hope that Bunny Ross is still alive.
"Certainly in their minds - and investigatively - there's been nothing to suggest he's not alive," he told the newspaper. "The other side of that is that he's been missing since 1977, and that's a long time to go without contact with family."