Bernard "Bunny" Ross, Jr., was last seen on May 12, 1977, after leaving his family's home in the town of Fort Kent. Police later found an abandoned vehicle they believe Ross had driven and dumped on a nearby dirt road.
No other trace of the 18-year-old man had ever been found, until an anonymous letter arrived.
Investigators said Ross' parents, Carol and Bernard, Sr., received the written communication a few months ago. Authorities have not released details of the letter, but said the author claims to have knowledge of the man's disappearance.
The bizarre new lead has investigators taking a fresh look into the case.
"I've never had anything like this happen in my career," Maine State Police Lt. Troy Gardner told the Portland Press-Herald. "Of course, there's no way of knowing whether the letter's truthful or the information is accurate."