Sited on an island just off San Francisco, Alcatraz - formerly a maximum security prison - is said to be one of the country's most haunted places.

The likes of Al Capone, Alvin Karpis (the first "Public Enemy #1") and Robert Stroud - the Birdman of Alcatraz' - were all inmates in the prison during its 29 years of operation.

And now it seems as if a pair of British tourists have accidentally photographed someone who never left Alcatraz when it closed in 1963…

Sheila Sillery-Walsh and her partner Paul Rice were visiting Alcatraz and snapping photographs, when they suddenly realised they'd caught more than they bargained for.

Speaking to The Metro, Sheila said: "When I glanced at the photo on my mobile, I saw this dark female figure in the picture. I looked at the window again and there was no one in the room."

She contacted prison staff, but none of them recognised the woman in the iPhone picture.

Being as Alcatraz only housed male prisoners, Sheila is convinced that she had captured an image of an inmate's female visitor.

She said: "I am so curious to know who she could be – perhaps she was a female visitor of a prisoner and kept returning.

‘I would love to know why she’s shown herself in my photo.

"Weirdly, when we were near that cell, a woman came on the audio tour who used to visit a prisoner. It makes you wonder."

Do you think the image offers proof of ghosts? Let us know your thoughts in the Comments Box below.