It was supposed to be the jail from which no inmate could escape - or if they were foolhardy enough to try, would surely perish in the attempt.

Alcatraz Federal Penitentiary sat on the eponymous island in the San Francisco Bay, 1.25 miles from the mainland and surrounded by strong, unpredictable currents.

A total of 36 people are known to have attempted to break out of Alcatraz, none are thought to have succeeded.

Yet the mystery around three inmates, brothers John and Clarence Anglin and their accomplice Frank Morris, who escaped in an infamous jailbreak in 1962, has never been conclusively solved.

The trio, immortalised by Clint Eastwood in the 1979 film Escape from Alcatraz, were long assumed to have drowned as they made their way from the island on a makeshift raft constructed from inflated raincoats.

Meanwhile some of the Anglins' descendants have maintained the men survived and have sporadically made contact with their families throughout the years.

Now it has emerged that Californian police recently received a letter decades after the FBI case was closed, purporting to be from one of the escapees looking to turn himself in to get medical care.

The letter

This week the Californian TV news channel KPIX 5 said it had obtained a letter sent to San Francisco Police Department’s Richmond station in 2013 and which prompted the FBI to reopen the cold case.

The writer claimed to be John Anglin and said the three inmates had "barely" made it through their escape, adding that Morris had died in 2008 and Clarence in 2011.

It then appears to propose an offer from Anglin to turn himself in for a minimal prison sentence in return for access to cancer treatment.

The letter reads: “My name is John Anglin. I escaped 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!”  

It added: “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 U.S. Marshals, the only agency still investigating the case today, said the FBI examined the letter for fingerprints, DNA, and the handwriting but the results were inconclusive.

Law enforcement agencies are still skeptical that the trio survived their attempt and went on to live incognito over the last five decades.

In a statement to KPIX 5, the U.S. Marshals Service said: “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.”

Yet the letter reopens the question: Did anyone ever successfully escape from the "escape-proof" prison?

Alcatraz

Alcatraz, whose name means ‘Island of gannets’, started life as a military fortress and prison in the 1850s and held Unionist deserters as well as Confederate sympathisers during the civil war.

It was acquired by the United States Department of Justice in 1933 and opened as a penitentiary in 1934. It soon became home to some of the most infamous criminals in American history such as Al Capone and Boston mobster, Whitey Bulger.

Contrary to popular perception, the facility was not explicitly a repository for the most dangerous criminals in America. Alcatraz was used more a destination for the most troublesome inmates in the correctional system, such as those who frequently caused fights or made multiple escape attempts.

For instance, Al Capone was moved to the island prison after accusations he had bribed prison guards in Atlanta to receive preferential treatment while serving his sentence for tax evasion.

Although the regime on the island was strict, especially in the early part of its history, it was actually seen as an relatively attractive destination by some convicts due to its private single-prisoner cells.

The 1962 escape helped expedite the prison's eventual closure in 1963 as it prompted an investigation that threw a spotlight on the ongoing maintenance and structural issues at the complex. Today it is a museum and major tourist attraction.

The Escape

Of the 36 inmates who attempted to escape the Rock, 23 were captured, six were shot to death and two were found drowned. The other five, including the Anglin brothers and Morris, were recorded as missing, presumed drowned. 

John and Clarence Anglin were born into a poor family of 14 children in Georgia and ended up in the US correctional system after pulling a series of bank robberies. The brothers generally attempted their heists when their targets were closed to avoid casualties and said they did used a weapon once - a toy gun.

John arrived in Alcatraz in 1960 after multiple prison break attempts with his brother and Clarence followed in 1961 for the same reason. While there the pair befriended Morris, a robber and drug dealer who had ended up on the island in 1960 after escaping from the Louisiana State Penitentiary before being recaptured a year later while committing a burglary.

Together the three hatched an audacious and intricate plan to escape the apparently inescapable penitentiary. Over six months they gradually worked to widen the ventilation duct in Morris's cell wall using sharpened spoons and discarded saw blades and then concealed the results with painted cardboard.

They also made an improvised drill from the motor of a broken vacuum cleaner and disguised the noise of their clandestine work by Morris playing his accordion. The trio even set up a makeshift workshop in the space in the roof of their cell to work on their illicit tools and hide materials they were stockpiling.

On the night of June 11, 1962, they crawled through the hole in their cell wall and then up a network of piles to the prison roof. They then slide down a smokestack and left the island on a raft made of more than 50 raincoats and inflated with a small concertina, which was used as an improvised bellows.

When the guards checked on their cells in the morning all they found were lifelike papier-mâché heads, with real hair from the barbers’ shop, and the beds stuffed with sheets.

June 11 was the last the trio were officially seen alive and they were presumed dead after a pile of bones later washed up on the bay coast was assumed to be their remains.

The FBI closed its investigation into the escape in 1979, saying: “We officially closed our case on December 31, 1979, and turned over responsibility to the U.S. Marshals Service, which continues to investigate in the unlikely event the trio is still alive.”

Could they have made it?

The key reason Alcatraz was considered escape-proof is the mile of churning water that surrounds it. If the three were not drowned through exhaustion in the frigid waters, the likelihood is that they would have been swept out to sea by the currents.

However that consensus was challenged in 2014 when a team of Dutch researchers used several interactive models recreate the conditions in the bay on the night of the escape.

The team concluded that the men had a real chance of survival if they disembarked during a short window between 11pm and midnight.

The study said if they all paddled hard the could have made it ashore, but if they left outside of that window the probability was high that they would have perished in the attempt.

Family believe they survived 

The descendants of the Anglin brothers have long maintained that they survived and sporadically made contact with their family over the intervening decades.

In 2015 two nephews of John and Clarence, Ken and David Widner, went public saying they thought their uncles were still possibly alive and in their 80s living in Brazil.

They said for three years John and Clarence’s mother received a Christmas card, signed by her sons. The handwriting was analysed and believed to be theirs – although the date of the cards could never be proven.

The family gave permission for the elder brother of John and Clarence, Alfred, to be exhumed and his DNA tested against that of the bones found on the bay coast - and they were found not to match.

Ken Wilder also said Whitey Bulger, who was in Alcatraz as the same time as the Anglins, wrote to him in 2014 saying he had coached them on how to avoid recapture if they ever escaped.

“He taught them that when you disappear, you have to cut all ties,” said Ken Widner. “He told me in a letter, ‘This is the mistake that I made.’

“He told me, ‘These brothers undoubtedly had done exactly what I told them to do.’ ”