Provincial police said firefighters found the body of a dog that had been with six-year-old Maxime Dion when he fell into the Riviere Noire on Monday.
But as darkness blanketed the area Wednesday the youngster was still unaccounted for.
"It was, of course, the dog we were looking for," Sgt. Daniel Thibodeau said.
"It was a couple of hundred metres from the spot where they were last seen."
The dog, along with Dion's older brother and a friend, tried to pull the boy from the water after he fell through the ice while playing along the shore.
The dog, which belonged to the friend, also ended up in the river during the rescue attempt, before vanishing in the frigid water.
On Tuesday, searchers found two boots — one is believed to belong to the missing boy and the other to his older brother.
Firefighters have been drilling holes in the ice on the river and police divers have been scouring the water for traces of Dion.
The Riviere Noire is about 80 kilometres east of Monteal.