Police say the six victims were newcomers to Canada, and the youngest was less than three months old.
A 19-year-old student from Sri Lanka who lived with the family has been charged in the killings.
“This was a senseless act of violence perpetrated on purely innocent people,” said Ottawa’s police chief on Thursday.
Officers responded to emergency calls placed at approximately 22:52 local time (03:52 GMT) on Wednesday from the Ottawa suburb of Barrhaven.
When they arrived, police chief Eric Stubbs said officers quickly identified the suspect, who was arrested without incident.
Officers then entered the home and discovered the victims, which included a mother, her four children, and an acquaintance who was living with the family.
The victims were identified as 35-year-old Darshani Banbaranayake Gama Walwwe Darshani Dilanthika Ekanyake and her four children.
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