Canadian police say a man driving a white car struck a police officer and stabbed him multiple times outside a football stadium Saturday.

Hours later, police say, the same man — this time driving a U-Haul truck — was pulled over a few miles away, drove off and struck four pedestrians.

The related attacks on Saturday night in Edmonton, the capital of Alberta, Canada, are being investigated as an act of terrorism, Edmonton Police Chief Rod Knecht said during a news conference early Sunday. He said an Islamic State flag was found in the passenger seat of the white Chevrolet Malibu the suspect was driving that night.

The suspect, a 30-year-old Edmonton man who has not been named, has been arrested. Knecht said the man acted alone, but investigators were not ruling out the possibility that others may be involved. The officer’s injuries were not critical, Knecht said, adding that he does not have any information about the other victims’ conditions.

The attacks began just after 8 p.m. during a football game at the Commonwealth Stadium. The officer was standing behind barricades and facilitating traffic outside the venue when the man rammed his Chevrolet Malibu into the barricades and struck the officer, sending him 15 feet into the air, Edmonton police said.

The man then got out of his car, “viciously” stabbed the officer with a knife and ran away, police said.

Less than four hours later, just before midnight, a U-Haul truck was pulled over about four miles away. An officer recognized that the driver’s name was similar to that of the registered owner of the Chevrolet Malibu, police said. The man immediately drove away, leading officers on a chase toward downtown Edmonton as he deliberately hit pedestrians on the way, police said.

The chase ended after the truck flipped over and landed on its side.

A witness, Austin Elgie, a manager of a bar in the downtown area, said the van “peeled” into an alley were people were smoking and struck a customer.

“There were like 10 cop cars following him … It was crazy,” Elgie told the Associated Press. “It just came around the corner, ripping. I thought at first he was pulling over for the cops coming by, but he was clearly the one they were chasing.”

Another witness, Pat Hannigan, told reporters that he watched officers pull out a man from the windshield of the wrecked van, the Associated Press reported.

Prime Minister Justin Trudeau called the attacks a “senseless act of violence.”

“Police officers put themselves at great personal risk every single day on our behalf, and this attack is a stark reminder of the sacrifices they make for the public good,” Trudeau said in a statement Sunday. “While the investigation continues, early reports indicate that this is another example of the hate that we must remain ever vigilant against … We cannot — and will not — let violent extremism take root in our communities.”

Investigators have not released any more information about the suspect. Knecht told reporters that police did not have any forewarning of the attacks.

He said Edmonton police are working with Canada’s Integrated National Security Enforcement Teams and other agencies.

The Canadian government does not track the number of terrorist incidents, though the country has a long history of such attacks. A research group has documented more than 1,400 terrorist attacks in Canada from 1960 to 2015.

Earlier this year, authorities charged a Canadian man with murder and attempted murder with a firearm after a deadly attack on a suburban Quebec City mosque. The January shooting rampage, which was called an act of terrorism, left six people dead and several wounded.

The attack is thought to be the first mass shooting at a mosque in North America.

In 2016, Canadian police killed a man who was planning to bomb an urban center during rush hour. Police, who were tipped off by U.S. authorities, caught the would-be suicide bomber as he was getting into a taxi carrying a backpack. The man detonated the bomb inside the taxi before police shot him.

In 2014, a gunman killed an honor guard soldier and opened fire inside the Parliament building in Ottawa. Days earlier, a “radicalized” Canadian convert to Islam struck two members of the Canadian military in a Quebec parking lot, killing one of them.

Washington Post writers Avi Selk, Carol Morello and Mark Berman contributed to this report.