Clarence and John Anglin and Frank Morris CBS San Francisco

SAN FRANCISCO — It is one of America’s greatest mysteries: What happened to three men after they pulled off a daring prison break at Alcatraz in 1962? Only the worst criminals were sent to Alcatraz. And for 29 years, it was the most secure federal prison in the country – surrounded by the cold, rough waters of the Pacific. But brothers John and Clarence Anglin and Frank Morris disappeared into the night and have never been found.

The men have become folklore – fueled by Hollywood and popular shows. And in the last 55 years, theories about their fate have multiplied as new evidence surfaces.

A letter allegedly written by one of the escapees recently came to light. CBS San Francisco exclusively obtained it from a source.

“My name is John Anglin. I escape 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 FBI says this is the most recent piece of evidence that forced the agency to reopen the iconic cold case. The letter was sent to the San Francisco Police Department’s Richmond station in 2013.

“It’s interesting, I mean it’s obviously a very famous case here in San Francisco,” said jeff Harp, a security analyst for CBS San Francisco.

Harp spent 21 years with the FBI, but did not work directly on this case.

“As a law enforcement person I’d like to think that their escape attempt was not fruitful for them. Personally, as someone who swims in the bay, and we have a triathlon that goes on every year, and there’s not a single person that doesn’t make that swim,” he added.

This past summer, we got an exclusive tour of some never-before-seen parts of Alcatraz. After months of meticulous planning – on the night of June 11, 1962 – the trio of bank robbers squeezed through the vents in the back of their cells.

The FBI says they…