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The mysterious disappearance of a 5-year-old boy and his grandparents is being probed by Canadian police. Nathan O’Brien's parents described their son as their "superhero" during an emotional press conference Wednesday in Calgary, Alberta. They also pleaded with any potential abductor to return Nathan. The boy's missing grandparents Kathryn and Alvin Liknes are not considered suspects in the case, according to the Calgary Police Department. Investigators say there were signs the trio had left the property against their will.

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The boy's mother Jennifer O’Brien contacted authorities when she found no one at home when she went to collect her son from his grandparents' house Monday morning. An Amber Alert remained in effect early Thursday but has not yielded any clues. Police have turned their attention to an estate sale held by the grandparents over the weekend. Officers are asking people who bought items to come forward so they can work out if any items went missing afterwards. “To whoever has Nathan, please find in your heart to drop off our little angel Nathan to which he can be rescued and reunited with his brothers and family,” the boy's father Rod O'Brien said during Wednesday’s press conference.

Image: Nathan O’Brien
Nathan O’Brien, 5, vanished along with his grandparents from their home in Calgary, Alberta.Calgary Police Dept.
- Alexander Smith