Two young siblings are in critical condition after they were struck by a vehicle that veered off a road in Vaughan on Sunday.
York Regional Police said they were called to Athabasca Drive, near Dufferin Street and Teston Road, around noon.
Officers said a black Mercedes Benz struck three people who were in a driveway, including an 10-year-old girl and her four-year-old brother. A male neighbour was also transported to hospital with minor injuries.
"It's such a devastating situation," Const. Laura Nicolle told reporters at the scene. "It is very, very serious in terms of the injuries."
She said before the crash, the children had been outside playing with their bikes when the neighbour noticed they had an issue with a bike chain and came over to help.
"They were on their own property and that's when the vehicle went off the road and collided with them," Nicolle said.
Nicolle said a 16-year-old boy behind the wheel was arrested and was alone in the vehicle.
Nicolle says the crash happened in an area where the speed limit was 40 km/h, so officers will be looking at whether speed was a factor in the collision.
"For something as horrendous and serious as this, we're not going to rule anything out until we know for sure," she said.
Investigators are appealing for any witnesses to come forward.