Police said that upon arrival, they located the two children with gunshot wounds in an apartment.
Paramedics said the boy and girl were transported to SickKids Hospital with serious injuries. Police added that the children were fatally injured.
Toronto police said a woman was in detention in connection with the incident. The authorities would not confirm the relationship, if any, between the woman and the children.
“We believe someone is in custody and responsible for the stab wounds,” said Constable Const. Alex Li told reporters Saturday afternoon.
“Having young children myself, I couldn’t imagine this type of trauma and injury,” said Li.
“This is a very serious stab and we are taking it very seriously and we have deployed our available resources to examine the matter … You can expect an active crime scene. “