Alcatraz federal prison in 1933. (AP)

During the night of June 11, 1962, Frank Morris and brothers John and Clarence Anglin slipped out of their cells at the infamous federal penitentiary on Alcatraz Island in San Francisco Bay and disappeared into our collective imaginations.

What happened to the trio of bank robbers? Law enforcement determined the men must have drowned in the bay's rough, cold water. The FBI closed its file on the escape more than 35 years ago, though the U.S. Marshals Service still considers it an open case.

But possible evidence keeps popping up -- as well as new theories -- that suggest the escaped criminals just might have made it to the mainland and lived long lives in unfettered anonymity.

Here's what you should know about the escape attempt:

Don't Edit

Top: Mug shots of Frank Morris, Clarence Anglin and John Anglin. Bottom: Photo-illustrations indicating what the men likely would have looked like years later. (U.S. Department of Justice)

How'd they do it?

"Over many months, Morris and the Anglin brothers chipped away at the wall around the vents in their cells, and at some point during the night of June 11, 1962, they pushed through the widened vents, made it outside, shimmied down a smokestack and set out on the water using a makeshift raft they'd fashioned out of raincoats," The Oregonian reported. "Their escape wasn't discovered until the next morning -- guards had been fooled overnight by plaster heads, complete with human hair, the escapees had positioned on their cots."

Don't Edit

AP

Once the escape had been discovered on the morning of June 12, federal and local officials responded with helicopters, National Guard troops and Coast Guard search vessels.

A fourth inmate, prolific car thief Allen West, planned to escape with Morris and the Anglins, but he had trouble getting out of his cell and eventually returned to his bunk. He died in prison in 1978.

Don't Edit

The girlfriend of one of the Anglin brothers may have been part of the scheme, with a plan to meet the escapees in Marin County and drive them to Mexico.

Hollywood immortalized the escape attempt with the 1979 movie "Escape From Alcatraz."

Clint Eastwood played Morris.

Don't Edit

AP

After the escape, the FBI quickly took control of the search and investigation.

"Our office in San Francisco set leads for offices nationwide to check for any records on the missing prisoners and on their previous escape attempts (all three had made them)," the bureau states.

"We also interviewed relatives of the men and compiled all their identification records and asked boat operators in the Bay to be on the lookout for debris. Within two days, a packet of letters sealed in rubber and related to the men was recovered. Later, some paddle-like pieces of wood and bits of rubber innertube were found in the water. A homemade life-vest was also discovered washed up on Cronkhite Beach, but extensive searches did not turn up any other items in the area."

Don't Edit

Don't Edit

The FBI determined that the escapees likely drowned and were swept out to sea. (A Norwegian freighter reportedly spotted a body in the ocean near San Francisco Bay shortly after the jailbreak.)

In 2014 a group of researchers using CARTO web-mapping tools analyzed tidal records for the night of the escape and concluded that Morris and the Anglin brothers might have survived if they'd pointed their makeshift rafts north (toward Marin County), paddled really hard and had more than a little luck.

The analysis of the June 11, 1962, tidal waves was a side project for the CARTO team: they are studying the potential impact of rising sea levels.

Check out their "Alcatraz Escape Simulation."

Don't Edit

AP

The FBI remains confident the escapees never made it to dry land.

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

Don't Edit

AP

The latest evidence

"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!"

This is how a handwritten 2013 letter to the San Francisco Police Department begins. The FBI lab reportedly tested the letter for fingerprints and DNA, and compared it to known handwriting by John Anglin. The results were inconclusive.

Don't Edit

The letter 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 FBI made no such announcement on TV, and the letter-writer hasn't been heard from since.

Don't Edit

AP

Could the letter be legit?

"It's always been talked about through the family," Anglin nephew David Widner said in January about the possibility the men survived the escape attempt. "My grandmother received roses for several years after the escape."

Don't Edit

Don't Edit

Whitey Bulger's Alcatraz mug shot. (AP)

During its heyday from the 1930s until its 1963 closure, Alcatraz was the place the federal government sent its most dangerous or difficult prisoners -- and especially those who persisted in trying to escape.

Well-known Alcatraz inmates over the years included Al Capone, Whitey Bulger, Meyer Cohen and Alvin Karpis.

Don't Edit

AP

At least 36 men tried to escape from Alcatraz. Other than Morris and the Anglin brothers, the only ones who might have been successful were Oklahoma criminals -- and accomplished prison escape artists -- Theodore Cole and Ralph Roe, who made it off The Rock in December 1937, never to be seen again.

"I doubt if even the most powerful swimmer could have survived those currents," Alcatraz's warden at the time, James Johnston, said of Cole's and Roe's disappearing act. "The tide would have swept them right out the Golden Gate into the sea."

And yet, notes the Tulsa World, "there were unconfirmed reports that Cole and Roe ... were picked up by a boat and were seen later in various locales ranging from Tulsa to Peru."

Don't Edit

AP

The former federal prison on Alcatraz Island is now a popular tourist attraction.