Calgary police fear for the safety of a five-year-old boy and his grandparents, who disappeared under “suspicious” circumstances and remain the subject of an Amber Alert.

Nathan O’Brien was staying with his maternal grandparents, Kathryn and Alvin Liknes, at their southwest Calgary home overnight Sunday. When his mother went to the house to collect her son on Monday morning, she found no one inside.

Duty Insp. Keith Cain said that while investigators are “optimistic” the three are still alive, investigators do fear for their safety.

“Given that this is completely out of nature for the grandparents, we look at it and say, ‘do we have reason to fear that foul play is involved here?’” Cain said. “And the answer to that question is obviously yes.”

Cain would not elaborate on any evidence found at the grandparents’ home. All he would say is that the evidence suggests the disappearances “may be suspicious.”

Earlier, Calgary Police spokesperson Kevin Brookwell said that when police entered the residence, they found that “the home was not in the condition that typically it would have been left in" and there were signs that the homeowners "did not leave on their own volition."

There are no custody issues with the family, Brookwell added. He said police do not have a history with the family. The family’s vehicles, including that of the grandparents, are accounted for.

The grandparents are not considered suspects, Cain said.

Investigators from several Calgary Police units are involved in the search, and many returned to the home Tuesday to continue looking for evidence. They have canvassed the neighbourhood, conducted a grid search and interviewed family members and associates, Cain said.

The family held an estate sale at the home over the weekend, he said. While police have yet to determine if it is connected to the disappearance, they want to hear from anyone who may have attended.

“Hopefully somebody remembers something from the weekend, or from Monday. Maybe they heard something,” Cain said. “We have a big mystery on hour hands right now. We don’t know where these people are.”

Neighbours say Alvin and Kathy Liknes had been planning to move away from Calgary.

"They seemed very relaxed and happy," neighbour Matt Dawe told CTV Calgary. "They were socializing with friends and family earlier on Saturday evening. Everybody sounded very relaxed and happy."

Police initially opted not to issue an Amber Alert because they did not have a description of a suspected abductor or a vehicle, Brookwell said. But they went ahead with the alert later, despite the missing criteria.

It is still unclear how the missing grandparents and their grandson left the home.

In an email to CTV News late Tuesday, Nathan's father, Rod O’Brien said his family will announce on Wednesday a reward in excess of $100,000 for the trio's safe return.

“Should get the world’s attention to get Nathan home,” he wrote.

Investigators are looking for:

Nathan O’Brien, 5, who is described as Caucasian, approximately 3-feet tall with blond, curly hair and brown eyes. He was last seen wearing peach-coloured shorts and a striped blue hoodie.

Alvin Cecil Liknes, 66, who is described as Caucasian, 6-feet-tall, 200 pounds, with a slim build and grey/blond hair.

Kathryn Faye Liknes, 53, who is described as Caucasian, 5-foot-3, 120 pounds with red/brown hair and green eyes.

Police are asking anyone with information about this case to call 403-266-1234 or Crime Stoppers anonymously by calling 1-800-222-8477, visiting calgarycrimestoppers.org, or texting tttTIPS to 274637.

With a report from CTV Calgary's Bill Macfarlane