Canadian police are questioning the suspected driver of a rented van that ploughed into pedestrians in northern Toronto on Monday, killing 10 and injuring 15.
Alek Minassian, 25, was not previously known to authorities, police said.
The incident appeared to be deliberate but the motive was not clear, officials added.
Canadian Prime Minister Justin Trudeau said the “tragic and senseless attack” had brought him “great sadness”.
Meanwhile, an officer has been praised for not opening fire during a tense standoff with the suspect, who claimed to be armed.
Video broadcast on CBC News showed a man pointing what appeared to be a gun at officers and shouting “kill me”.
The officer tells the man to “get down” and when the suspect says he has a gun, the officer repeats: “I don’t care. Get down.” The suspect was then arrested without shots being fired.
Minassian is due to appear in court at 10:00 local time (14:00 GMT) on Tuesday.