According to Sheila Sillery-Walsh, she photographed a ghost.
In the image, a woman appears to be looking directly through a window at the top of a door.
"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," Sillery-Walsh told the Daily Mail. "From that point onwards, I wasn't interested in the Alcatraz tour anymore. I just kept looking at the picture over and over again."
Sillery-Walsh has been trying to identify the woman in the photo by contacting former Alcatraz staff members, but the effort has been unsuccessful thus far.
"I am so curious to know who she could be though -- perhaps she was a female visitor of a prisoner who kept returning back," Sillery-Walsh said. "I would love to know why she's shown herself in my photo."
Former Alcatraz prison guard George DeVincenzi told KPIX 5 that he thought the image in the photo was just shadows. "Personally I don't believe in ghosts," he said.