Police in Canada were compiling eyewitness accounts and other evidence Tuesday to understand why a driver jumped the curb of a bustling intersection in downtown Toronto, killing 10 people and injuring more than a dozen others in the worst mass killing in the country in three decades.
The suspect — 25-year-old Ontario man Alek Minassian — was set to appear in court Tuesday morning, with charges being released at that time, police said. He was arrested and taken into custody following the chaos Monday afternoon in Canada's largest city.
Toronto Police Chief Mark Saunders said hours after the incident that the motive for the ramming remained unclear, but "definitely looked deliberate."
"We are looking very strongly to what the exact motivation was for this particular incident to take place," Saunders said.
Law enforcement officials in Canada and the United States who have been briefed on the case told NBC News that the leading theory appears to be mental illness and not terrorism, although that can change.
Canadian Public Safety Minister Ralph Goodale was in Toronto on Monday for a meeting of G-7 security ministers. He later told reporters that the killings didn't appear to be related to terror.
Authorities provided no details about Minassian and said he was not previously known to police. The law enforcement officials said he was once involved in an online discussion about Elliot Rodger, the 22-year-old gunman who killed six people in 2014 near Santa Barbara, California.
Witnesses of the Toronto rampage, which occurred at about 1:30 p.m. ET, described a white van apparently rented from Ryder swerving back and forth between the sidewalk and the road, and moving through crowds at 30 mph. Besides those killed, police said 15 people were also injured.
Videos shared on social media show a brief sidewalk standoff between the suspect and a lone police officer after he got out of the van:
"C'mon, get down!" yells the officer, whose weapon was drawn, in the video. "Get down or you'll be shot!"
"I have a gun in my pocket!" the driver responds. "Shoot me in the head!"
The suspect was not found in possession of a gun, police said.
Prime Minister Justin Trudeau, who was expected to speak about the incident Tuesday morning, called the attack "senseless and tragic."
Canada's threat level has been at "medium" since October 2014, when a man gunned down a Canadian soldier at the National War Memorial in Ottawa before he was shot and killed by the Parliament's Sergeant-at-Arms.
The car attack follows several other deliberate ones around the world — including in New York, Spain and France — that were connected to or at least inspired by terrorism.