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A suspect who allegedly attacked a police officer with a knife and later drove a U-Haul truck into pedestrians in Edmonton, Alberta is being investigated for "acts of terrorism," police in the Canadian city said early Sunday.

The 30-year-old man was detained after a high-speed chase through streets filled with Saturday night revellers and local football fans.

Edmonton Police Chief Rod Knecht told reporters that the suspect was in custody and they think he acted alone, but they aren't ruling out the possibility that others were involved.

The U-Haul flipped during a police chase. Michael Mukai

An apparent ISIS flag was seized from the passenger seat of the truck, he confimed.

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“We believe the individual acted alone," Knecht said. He confirmed that an ISIS flag was seized from the vehicle initially connected with the attack on the police officer.

The chaos began outside the Edmonton Eskimos football game at Commonwealth Stadium at 8 p.m. local time (10 p.m. ET) Saturday when police say a vehicle rammed a traffic control barricade and sent an officer flying into the air.

Knecht says the driver of the vehicle then got out and “viciously” stabbed the officer several times with a knife before fleeing.

The officer was taken to hospital and treated for non life-threatening injuries.

At around midnight, a U-Haul van was stopped at an impaired driving stop north of downtown and the driver sped off with police in pursuit, Knecht said.

The U-Haul hit and injured four pedestrians before it flipped over. Their condition was not immediately known.

Knecht said the apparent ISIS flag would be "part of the investigation," adding that there had been no prior warning of a specific attack.

Witnesses say Uhaul swerved down Jasper with at least 15 police cars in pursuit, turned down 107 Street. Reports of shot fired by police pic.twitter.com/PwuUhvWHHh — Zoë Todd (@ZoeHTodd) October 1, 2017

Witness Brian McNeill told CBC he was with a group of friends in Boston Pizza at 106th Street when the van tore past on the wrong side of the street pursued by police.

"That high-speed chase should have never, ever in a million years have happened downtown," he told the station. "If you've got 20 cops chasing one guy and there's pedestrians that are walking down this road?"

The suspect was injured but not critically, Knecht said.