Canadian police said on Sunday that an attack on an officer outside a football game with a car and knife and a high-speed chase of a U-Haul that left four people injured in the western city of Edmonton, Alberta, are being investigated as acts of terrorism.

Edmonton Police Chief Rod Knecht said that officers took a suspect in custody and they think he acted alone.

Knecht said a flag of the militant Islamic State (IS) group was found in the car that hit the officer. They later described the suspect as a 30-year-old Edmonton man.

The attack began on a Canadian Football League game outside Commonwealth Stadium on Saturday night when police say a white Chevrolet Malibu rammed a traffic control barricade and sent an officer flying into the air 15 feet.

Knecht said the driver, believed to be 30 years old, then got out and attacked the officer with a knife before fleeing on foot.

The officer was taken to a hospital and treated for non-life threatening injuries while a manhunt was launched. “It's not critical,” Knecht said of the injuries.

A few hours later, a U-Haul van was stopped at an impaired driving check stop north of downtown on Wayne Gretzky Drive. Knecht said the name of the driver was close to the name of the registered owner of the car that hit the officer.

He said the U-Haul then sped off toward downtown with police in pursuit.

Police say the U-Haul intentionally swerved at pedestrians at crosswalks throughout the chase. Four people were injured by the van, but the extent of their injuries was not immediately known.

The van eventually flipped near a downtown hotel and a suspect was arrested. Knecht said the man was known to police, but did not release his name.

“It is believed at this time that these two incidents are related,” Knecht said.

“It was determined that these incidents are being investigated as acts of terrorism.” Knecht said Edmonton police are working with the Royal Canadian Mounted Police's National Security Enforcement Team and other Canadian security agencies. Knecht told the public to remain vigilant and observant of their surroundings.

Knecht said police didn't call off the chase of the U-Haul due to the seriousness of the crime.

Witness Pat Hannigan told reporters that he saw police pull the man from the windshield of the toppled U-Haul. He said 30 police cars were chasing the U-Haul.

Alberta Premier Rachel Notley said on Twitter that her thoughts are with the injured officer and she's hoping for a speedy recovery.