And, if so, are they still alive, nearly 56 years later?

To this day, Frank Morris, Clarence Anglin and John Anglin remain the only people who have escaped Alcatraz and never been found — a disappearance that is one of the country’s most notorious unsolved mysteries.

The prevailing theory is that Morris and the Anglin brothers drowned after leaving Alcatraz Island and attempting to cross the frigid San Francisco Bay.

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But in a newly surfaced letter sent to San Francisco police in 2013 and obtained by CBS affiliate KPIX, a man claiming to be one of the escapees said that all three of the prisoners survived the attempt — but that he was the only one still living.

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“My name is John Anglin,” the handwritten letter began. “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 claimed that Morris died in 2008 and that Clarence Anglin died in 2011.

The note 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.”

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That was nearly five years ago.

A U.S. Marshals Service representative told The Washington Post that the agency believes the letter is without merit. According to the agency, the letter was submitted to an FBI lab for forensic handwriting analysis, comparing it to samples from all three escapees, and the results were “inconclusive.”

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“At this time, there are no leads stemming from the 2013 anonymous letter,” the Marshals Service said in a statement.

It’s unclear why it took nearly five years for the letter to surface. FBI officials did not immediately respond to requests for additional details Wednesday morning.

Whatever the fate of the escapees, the bold prison break from a maximum-security facility nicknamed “The Rock” has become the stuff of legend, immortalized in the 1979 movie “Escape from Alcatraz,” which starred Clint Eastwood.

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According to the FBI, this much is known: About six months before the escape attempt, four Alcatraz prisoners began plotting their jailbreak, using found and stolen materials (including a broken vacuum cleaner motor) to fashion a makeshift drill.

Slowly, each of the men drilled tiny holes around the air vents in the back of their cells, then punched out a portion of the wall large enough to wiggle through.

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For weeks, the FBI stated, the prisoners used the air vents to access an empty corridor they used as a “secret workshop” to build their escape equipment, which attested to the men’s ingenuity:

More than 50 raincoats that they stole or gathered were turned into makeshift life preservers and a 6×14 foot rubber raft, the seams carefully stitched together and “vulcanized” by the hot steam pipes in the prison (the idea came from magazines that were found in the prisoners’ cells). They also built wooden paddles and converted a musical instrument into a tool to inflate the raft. At the same time, they were looking for a way out of the building. The ceiling was a good 30 feet high, but using a network of pipes they climbed up and eventually pried open the ventilator at the top of the shaft. They kept it in place temporarily by fashioning a fake bolt out of soap.

At last, sometime between the evening of June 11 and the morning of June 12 in 1962, Morris and the Anglin brothers made their escape, slipping out through the air vents in their cells one last time, grabbing the equipment from their secret workshop and climbing up to the ventilator onto the prison roof. They were able to trick night guards into thinking they were still sleeping by tucking dummy heads into their beds before they left.

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They were never seen again.

According to the FBI, Allen West, one of the four men originally in on the plan, wasn’t ready in time and was left behind. West died in 1978.

The agency used interviews with him to glean most of the details about the plans leading up to the escape — but evidence after the escape remained scant, save for pieces of homemade paddles and a life vest that washed ashore nearby.

For decades, questions have persisted: Did the men successfully make it north toward Angel Island, either in their makeshift raft or by swimming? Or were they overcome by the bay’s rough waters, their bodies long lost to the Pacific Ocean?

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The unsolved case has spawned everything from television specials to interactive modeling of current conditions the night of the escape.

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Alcatraz closed as a prison in March 1963, less than a year after the infamous escape, but the island facility remains one of the San Francisco Bay area’s best-known tourist attractions — and the starting point for the grueling “Escape from Alcatraz” triathlon.

The FBI officially closed its case on the Alcatraz escapees in 1979.

“For the 17 years we worked on the case, no credible evidence emerged to suggest the men were still alive, either in the U.S. or overseas,” the FBI stated.

However, the Marshals Service has continued to investigate leads and said it will do so until the men are proven deceased, or until they turn 99.

“The ongoing U.S. Marshals investigation of the 1962 escape from Alcatraz federal prison serves as a warning to fugitives that regardless of time, we will continue to look for you and bring you to justice,” Marshal Don O’Keefe of the Northern District of California said in a statement in 2012, the 50th anniversary of the escape.

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Relatives of John and Clarence Anglin firmly believe they survived their escape: At least four members of the Anglin family, including two nephews and a sister, spoke to the San Francisco Chronicle in 2013, furnishing what they said was evidence the men were alive — including a Christmas card the family received in 1962 that read: “To Mother, from John. Merry Christmas.”

“If they are not alive,” a nephew, Dave Widner, told the newspaper, “then why is the government still looking for them?”