A letter emerged Tuesday that was purportedly written by a former inmate at Alcatraz who — along with two others -- managed to escape the island prison only to vanish without a trace.

The running theory about the inmates' fate is that they died shortly after stepping foot into the cold waters that separated the prison and San Francisco. But their bodies were never found and their story remains a mystery.

Prison officials and federal agents insisted at the time of the escape that the inmates — brothers John and Clarence Anglin and Frank Morris — perished.

CBS San Francisco reported that it obtained a letter allegedly written by John Anglin. The letter contains an admission of escape and an explanation of the inmates' fate.

“My name is John Anglin,” the letter reads. “I escape (sic) from Alcatraz in June 1962 with my brother Clarence and Frank Morris. I’m 83 years old and in bad shape. I have cancer. Yes we all made it that night but barely.”

The letter continued, “If you announce on TV that I will be promised to first 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. This is no joke.”

The letter was sent to the San Francisco Police Department’s Richmond station in 2013, the report said. The station said it obtained the letter from an unnamed source.

The FBI tested the letter in 2013 for fingerprints but reportedly said results were inconclusive.

The three prisoners were serving sentences for bank robbery when they executed their escape plan using stolen spoons, dummy heads and a raincoat raft. Their exploits were turned into the 1979 movie “Escape from Alcatraz,” starring Clint Eastwood as Morris.

U.S. Marshal Michael Dyke, who inherited the unsolved case in 2003, told The Associated Press in 2012 that he didn’t know whether any members of the trio were still alive. But he had seen enough evidence to make him wonder.

That evidence included credible reports that the Anglins’ mother, for several years, received flowers delivered without a card, and that the brothers attended her 1973 funeral disguised in women’s clothes despite a heavy FBI presence.

The report pointed out that today Morris would be 90 and John and Clarence, the brothers, would be 86 and 87.

The federal government closed Alcatraz as a prison in 1963, just a year after the men's escape.

John Cantwell, National Park Service ranger, told the station that the Federal Bureau of Prisons said they drowned “once they got off” the island and were swept out to the Pacific.

“End of story,” he said.

The Associated Press contributed to this report.