Five students were stabbed to death in a rampage at a house party in Calgary, Canadian authorities have said.
Police said they found three people dead after being called to a home in a suburban neighbourhood at about 1.20am (8.20am Irish time).
Two more students died in hospital after being attacked at the party to mark the end of university classes.
The victims were four men and one woman in their 20s.
One of them was discovered wounded on the front lawn of the property.
A suspect was arrested nearby after what authorities said was a chase involving police dogs.
The 23-year-old suspect is a former University of Calgary student, and the son of a senior police officer, public broadcaster CBC reported.
The victims were also students at the university, with authorities saying there were as many as 30 people at the party.
Calgary Police Chief Rick Hanson called the incident the "worst mass murder" in the city's history.
"We have never seen five people killed by an individual at one scene. The scene was horrific," he told a nationally televised news conference.
He said the suspect was an invited guest who had headed to the gathering after work at a local grocery store.
"The suspect arrived at the party, obtained a large knife and targeted the victims one by one, stabbing them several times," Chief Hanson added.
However, "at this point in time, we can't say why."
The University of Calgary said it was "greatly concerned about students, faculty and staff affected by this tragedy".
Calgary Mayor Naheed Nenshi tweeted that the "thoughts and prayers of all Calgarians are with the young people we lost this morning, their families, friends, and university community".
Canadian Prime Minister Stephen Harper also offered his condolences.