“My name is John Anglin,” reads the letter. “I escape from Alcatraz in June 1962. Yes we all made it that night, but barely!”
The breakout was immortalised in the 1979 Clint Eastwood movie Escape from Alcatraz.
What does the letter say?
It claims brothers John and Clarence Anglin along with inmate Frank Morris lived into old age after escaping the prison half a century ago.
The author says Clarence Anglin died in 2008 and that Morris died in 2005.
The writer tries to make a deal with authorities, saying: “If you announce on TV that I will be promised to just go to jail for no more than a year and get medical attention I will write back to let you know exactly where I am.
“I am 83 years old and in bad shape. I have cancer.”
According to the letter, John Anglin lived in Seattle for most of his life and spent eight years in North Dakota.
At the time the letter was sent he was reportedly living in southern California.
Is the letter genuine?
San Francisco police did not make the letter public despite obtaining it five years ago, according to CBS.
To this day, the three escapees remain on the most wanted list with law enforcement photos of what they might look like today.
The US Marshals Service, which has been responsible for the case since 1978, submitted the letter to the FBI laboratory for forensic handwriting analysis.
“Handwriting samples of all three escapees, John Anglin, Clarence Anglin and Frank Morris, were compared to the anonymous letter, and the results were deemed ‘inconclusive,'” said a statement from the US Marshals Service.