Thirty-six inmates tried to escape the Alcatraz Federal Penitentiary, and as far as the authorities are concerned, they all failed. However, the trio of convicts that came closest to success was that of Frank Morris, John and Clarence Anglin.

The inmates worked at night over a period of six months, using discarded saw blades, stolen spoons, and a drill made from the motor of a vacuum cleaner to widen the ventilation ducts in their cells. Morris covered the noise by playing his accordion and the holes were covered with cardboard and paint.

They climbed on top of the cell block and set up a workshop, turning 50 raincoats into life preservers and a rubber raft, stitched together and heat-sealed with steam from the pipes, making paddles from scrap wood.

Eventually, escaping through the roof, their disappearances were hidden using dummy heads made from a paper-mâché-like mixture of soap and toilet paper, paint from the maintenance shop and hair from the barbershop floor.

They descended 50 feet by sliding down a pipe, then climbed over two barbed wire fences to a blind spot in the prison's searchlights, where they inflated their raft and took to the water.

But from there, things get a bit fuzzier. The FBI's search found only remnants of the raft, and they decided that they had likely drowned. However, there have been some hints that they got away safely.

In 2012, the Anglins' family claimed that the brothers were alive, but had kept a distance due them having to "cut all ties". One sibling had told family members on his deathbed that he had been in contact with the brothers from 1963 until 1987, suggesting that this photo shows the brothers alive and well in 1975:

Now, a letter from one of the men who escaped has lent further validity to this theory. After just over 50 years, the FBI received a letter in the mail claiming to be from John Anglin, explaining that he and the others did survive. The letter was delivered in 2013, but it has only just been revealed to the public, and reads:

"My name is John Anglin. 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! 'Frank passed away in October 2005. His grave is in Alexandria under another name. My brother died in 2011.

"If you announce on T.V. 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 this is for real and honest truth."

The FBI inspected the letter to see if there were any fingerprints or whether the handwriting could be matched, but all tests were inconclusive.

Brothers John and Clarence Anglin were sentenced to 15-20 years behind bars for robbing banks and other establishments. They served their time in three different prisons, but after repeated escape attempts, they were sent to Alcatraz to make sure they wouldn't break out again.

"There is absolutely no reason to believe that any of them would have changed their lifestyle and became completely law-abiding citizens after this escape," A US Marshall's spokesperson told KPIX 5.

Now, if this letter is genuine, it could mean that the infamous 1962 Alcatraz escape was actually pulled off. In the meantime, check out this list of other daring prison escapes, and how they did it.