The letter, about Bernard Ross Jr., who was 18 when he disappeared in 1977 after leaving the family's home in Fort Kent, arrived at the parents' home "a couple of months ago," according to The Portland Press Herald.
The police disclosed the existence of letter, hoping that public attention might coax the author of the correspondence to contact police.
The letter to Ross' parents referred to a report about him published in The Kennebec Journal.
Maine State Police Lt. Troy Gardner said that authorities are not releasing the contents of the letter and that the family is going through a wide range of emotions after being in touch with police throughout the years.
As for whether the letter is authentic, Gardner told ABC News today, "that"s the question. We don't know if it's a hoax. If it is a hoax, it's just a terrible thing to do to a family."
A web page detailing information about Ross by the Charley Project, an independently run nonprofit group devoted to locating missing people, says he was last seen on Realty Road in Ashland, Maine.
He was "despondent at the time of his disappearance,” the project says.
Meaghan Good, who runs the Charley Project website as a volunteer, told ABC News that the details in her report were cobbled from a number of sources, including the Center for Missing and Exploited Children, a private nonprofit; tips on Websleuths, a web forum devoted to amateur crime solving; and old newspaper clippings from The Daily Bangor News, a paper focused on rural Maine.
She said her site has helped families locate other missing people, including a man who disappeared from New Mexico in 2004.
ABC News has been unable to reach Ross' parents.