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.
Police said the white rental van mounted the kerb on Yonge Street between Finch Avenue and Sheppard Avenue at about 13:30 local time (17:30 GMT) on Monday and drove into pedestrians along a 2km (1.24 mile) stretch.
Reza Hashemi, who owns a video shop on Yonge Street, told the BBC he heard screaming on the other side of the road.
He said the van was repeatedly mounting the pavement and running into people.
One witness told City News that the driver was "hitting anything that comes in the way".
"People, fire hydrants, there's mail boxes being run over," said the unnamed man, who said he was driving behind the van during the incident.
As the van continued, the man said he sounded his horn to try to warn pedestrians. "I witnessed at least six, seven people being hit and flying in the air, like killed, on the street," he said.
Pictures from the scene showed bodies covered in orange sheets along the van's route. Debris and items of clothing were scattered across the pavements and road.
The van was brought to a halt by police several streets away and was quickly surrounded.
Police said Alek Minassian was from the northern Toronto suburb of Richmond Hill and was not previously known to authorities.
"The actions definitely looked deliberate," said Toronto Police Chief Mark Saunders.
Public safety minister Ralph Goodale said there "would appear to be no national security connections" and Canadian broadcaster CBC cited government officials as saying said Mr Minassian was not associated with any known terror groups.
Minassian had previously attended a school for students with special needs in north Toronto, former classmates said.
He would be seen walking around Thornlea Secondary School with his head down and hands clasped tightly together making meowing noises, Shereen Chami told Reuters.
But she said Minassian had not been violent.
"He wasn't a social person, but from what I remember he was absolutely harmless," she told Reuters.