Officers with the Calgary Police Service are in Mexico as part of their investigation into the disappearance of five-year-old Nathan O'Brien and his grandparents Alvin and Kathy Liknes, CBC News has learned.

The three, who are presumed to be dead, have not been seen since the end of June when they were discovered to be missing from a home in southwest Calgary where a violent incident took place.

Douglas Garland

Douglas Garland is escorted into a Calgary police station in connection with the disappearance of Nathan O'Brien and his grandparents. (CBC)

No bodies have been recovered, but police have said there is evidence that the three are dead.

Last month, 54-year-old Douglas Garland, who was previously considered a person of interest in the case, was charged with two counts of first-degree murder and one count of second-degree murder.

Garland’s sister, Patti, is in a common-law relationship with Alvin Liknes's son.

Police have already conducted an extensive search of a rural property near Airdrie, north of Calgary, owned by Garland’s parents.

Police have not said exactly where the officers are or what they are looking for in Mexico.