Chief Rick Hanson says a man was arrested early Monday but he can't identify him because the charges were not going to be laid until later in the day.
Nathan O'Brien and his grandparents, Kathryn and Alvin Liknes, haven't been seen since June 29 after what police say was a violent incident in the Liknes house.
Hanson says their bodies have not been found, but investigators have enough evidence to conclude that they are dead. An Amber Alert issued shortly after the family members were reported missing has been cancelled.
Shortly after the three disappeared, Nathan's parents issued a tearful plea through the media asking for their return and urging their "superhero" son to be brave.
Hanson says that hope is now gone.
"Even as the days went by there's always a hope, there's always a glimmer of hope. And unfortunately with the laying of the charges, we've taken that hope away from the family. So they are devastated."
He says the case is not closed and investigators will continue to search for their bodies. He asked rural property owners to search their land for anything that appears out of the ordinary.
More than 200 officers and analysts worked on the complex and heart-breaking case, he says. There were about 900 tips from the public.
The chief says the man arrested is from Airdrie, a community north of Calgary, and investigators are not looking for any other suspects.
He would not confirm that the suspect is Douglas Garland, who lives on an Airdrie acreage with his parents and has been identified as a person of interest in the case.
Garland was first questioned more than a week ago and was released from custody last Friday on unrelated identity theft charges.
Media reports citing police sources say it's Garland who has been re-arrested.
Garland, who is 54, has ties to the Liknes family.
Police have said his sister is in a relationship with a family member and there have been reports he had business dealings with Alvin Liknes as well.
Investigators have been searching the Garland acreage northeast of Airdrie where since he was initially questioned.
Over the past two weeks RCMP and Calgary police have searched several fields, sloughs and stands of trees on the acreage and in an area nearby.