The remains of Gordon Klaus, 61, and his daughter Monica Klaus, 40, were found in December in the rubble of their burned-out farmhouse near Castor, a small town east of Red Deer.
The man's wife, 62-year-old Sandra Klaus, has not been located, but RCMP suspect she was also killed.
"Our investigators are most certain that she was a victim of a homicide as well," RCMP spokeswoman Sgt. Josee Valiquette said at a news conference Thursday.
It's possible the woman's body could have been destroyed in the fire, she said. Investigators have determined the blaze was deliberately set, although they're not revealing how it started.
Valiquette said the home was heated by coal and there was a significant amount of the fuel in the home's basement, which made the fire hot and its damage severe.
"It was very intense and the remains are not necessarily easy to analyze."
Emergency responders found the family's dog shot dead outside the home.
Valiquette could not say if the victims were also shot, or if they died before the home was set on fire.
She also wouldn't divulge whether investigators have any suspects, but she did confirm the family members were targeted.
"We don't believe that this was a random act of violence."
Jason Klaus, 38, lived in a home across from his parents but was unharmed. He told media that his sister lived in Stettler and had been visiting their parents that weekend.
On Dec. 11, three days after the fire, police found the family's white GMC Sierra truck abandoned about 30 kilometres away.
Valiquette said officers are seeking help from anyone who may have seen the pickup truck on the night of Dec. 8.