Image caption Frank Lee Morris, Clarence Anglin and John Anglin (l-r) made it out of Alcatraz, but did they survive?

Those who ran Alcatraz liked to say nobody ever escaped alive.

But that hasn't stopped US marshals from continuing the search for three men who made it off the island 50 years ago. According to the official version, Frank Morris, and the brothers John and Clarence Anglin were presumed drowned in the cold and choppy waters of San Francisco Bay.

There are plenty of people who think they did make it ashore and have been in hiding ever since.

Rumour had it they would return to the prison turned tourist spot on the 50th anniversary of their escape. Although it's not certain where the urban myth began, US Marshal Michael Dyke spent the day on the island anyway, just in case.

Papier-mache heads

Most prisoners who tried to flee The Rock, as it became known, were captured, killed or drowned.

But this was one of the most daring and intricate escapes in the notorious prison's history - involving spoons, papier-mache heads and rubber raincoats.

It began by digging away at the concrete around the air vents in their cells with spoons and a drill-like device fashioned from a vacuum cleaner.

Image caption Alcatraz was closed in 1963, a year after the escape

Accordion practice muffled the sound of the drilling, and cardboard was carefully used to cover each hole as it grew. Soap became a substitute for removed rivets.

When the time came, they squeezed through into a utility corridor and headed for the roof.

Guards doing their rounds periodically checked on the faces of their prisoners. The three escapees appeared to be sleeping soundly, the guards were unaware they were papier-mache heads with real hair, harvested from the prison barber shop.

The three made it up to the roof, and despite the searchlights, headed over high barbed-wire fences.

At a watchtower blind spot they used improvised bellows to inflate a raft fashioned from rubber raincoats.

A fourth member of the gang had been unable to remove his air vent quickly enough, and by the time he broke through, the others had already cast off into San Francisco Bay - to their deaths or to freedom - depending on what you believe.

Remnants of the raft were found washed up on a nearby island, but the men were never seen again.

Open case

"I think there's a good possibility that they survived," US Marshal Michael Dyke says.

"It's hard to say. We have to keep the case open since no bodies have been found, but about a month after they escaped in July 1962 a Norwegian freighter saw a body floating in the ocean 15 miles from the Golden Gate Bridge.

"He had on prison clothes - a navy pea coat and a light pair of trousers - similar to what prisoners wore. There were no other missing people during that time period."

He thinks that may have been the body of Frank Morris, believing the Anglin brothers would have looked after each other.

Media playback is unsupported on your device Media caption Phil Dollison was living on Alcatraz island when three men escaped

But the uncertainty over their fate created a legend. Books and documentaries continued to question whether they drowned, or in fact made it to shore. Clint Eastwood played Frank Morris in the 1979 film Escape from Alcatraz.

One TV show re-enacted the escape in similar conditions and concluded they could have survived.

"I still receive leads once in a while regarding the case and there are still active warrants," said Mr Dyke says. He has personally been investigating for almost 10 years.

"Because it's an open case we have to go looking for them. Most leads aren't really that good or credible. Generally 99 percent aren't true."

But along with a few relatives of the missing men, he went to the island for the day prepared to make an arrest if necessary on the 50th anniversary.

"Rumours start somewhere and nobody knows where they come from. There's always a legend, but I don't think it's going to happen."