07/02/2014 11:10 EDT

Nathan O'Brien, Grandparents Still Missing As Amber Alert Remains Active

An Amber Alert remained in effect across Alberta on Wednesday as police continue to search for leads in the perplexing disappearance of a five-year-old boy and his grandparents.

Nathan O'Brien was reported missing Monday morning when his mother went to pick him up after a sleepover at the grandparents' southwest Calgary home and they weren't there.

Police say based on things they found in the home, they don't believe the residents left voluntarily.

Also missing are O'Brien's grandparents, 66-year-old Alvin Cecil Liknes and Kathryn Faye Liknes, 53, who are not suspects.

Nathan's father, Rod O'Brien, made an emotional appeal Wednesday that was carried live on national TV.

"Nathan, we are going to bring you home very soon,'' O'Brien said. "Nathan, God is taking care of your soul and Mom and Dad need you to be brave right now.

"Whoever has Nathan, please find it in your heart to drop off our little angel Nathan (so) he can be rescued and reunited with his brothers and his family.''

Investigators are asking anyone who attended an estate sale at the residence in the 100 block of 38A Avenue S.W. over the weekend to visit the Parkhill Community Centre on Thursday, July 3, between 10 a.m. and 10 p.m.

Those who purchased something are asked to bring a photo of that item to help detectives determine what items were bought during the sale, compared to what items remain in the home.

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According to police, officers in the Liknes' neighbourhood have interviewed neighbours, family members and friends of the family, and also performed a grid search of the area, but have no significant leads.

"We have a big mystery on our hands right now. We don't know where these people are. We are still very optimistic that they are alive, and we are doing everything we can to locate them, but we need the help of the public," Duty Insp. Keith Cain told reporters Tuesday afternoon.

Calgary police spokesman Kevin Brookwell told CTV News there are no custody issues with the family and that all of the family's vehicles are accounted for.

Nathan’s father, Rod O’Brien, told the Calgary Sun a reward consisting of a “large sum” would be announced Wednesday.

According to CTV News, the reward could be in excess of $100,000.

Nathan O'Brien is described as three feet tall, curly blonde hair, brown eyes, and was last seen wearing peach-coloured shorts and a blue-striped hoodie.

Alvin Liknes is six feet tall, 200 pounds with grey and blond hair and was wearing black shorts. Alvin has a distinctive walk.

Kathryn Liknes is five-foot-three-inches tall, 120 pounds with reddish-brown hair and green eyes.

Anyone with information about the case should phone police at 403-266-1234 or call Crime Stoppers.

With files from the Canadian Press