Photo: CTV

Edmonton police believe an unprovoked knife attack on an officer controlling traffic outside a football game and a high-speed chase of a moving van that struck and injured four pedestrians were acts of terrorism.

At a news conference early Sunday morning, Chief Rod Knecht said police have a 30-year-old man in custody and they think he acted alone. But Knecht stressed it is early in the investigation and they aren't ruling out that others were involved.

An Islamic State militants flag was seized as evidence, he said.

"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."

The chaos began Saturday night outside the Edmonton Eskimos CFL football game at Commonwealth Stadium.

It was military appreciation night. Canada's chief of defence staff, Gen. Jonathan Vance, conducted the pregame coin flip and two CF-18 fighter jets did a fly-past before kickoff. More than 800 Boy Scouts were expected at the game and many were planning to camp out on the field afterward.

While the Eskimos were battling the Winnipeg Blue Bombers inside the stadium, outside a white Chevy Malibu approached a traffic control post at a high rate of speed.

Knecht said the car rammed through a barricade, throwing an officer about five metres into the air, then crashed into a police cruiser.

The driver hopped out and began "viciously" stabbing the officer with a knife before running away, Knecht said.

A manhunt was launched as the officer was taken to hospital and treated for non life-threatening injuries.

Knecht said the Islamic State flag was found in the front seat of the car.

A few hours later, while fans filed out of the game and were re-routed around the crime scene, a U-Haul cube van was stopped at a checkstop north of downtown.

When the driver was asked for his licence, Knecht said the name on the identification was close to that of the registered owner of white Malibu.

When confronted, Knecht said the U-Haul sped off toward downtown with police cars in pursuit.

The van intentionally swerved at pedestrians in crosswalks, Knecht said. Four people were injured by the van. The extent of their injuries was not immediately known.

Austin Elgie, manager of The Pint bar just west of the downtown core, saw the van zoom by with police giving chase.

The van "peeled" into an alley where people were smoking, he said.

"There were like 10 cop cars following him ... It was crazy. 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."

Elgie said the van hit a man who was a bar customer.

"I have a registered nurse on my bar team and I grabbed her and had her look after the guy until the ambulance came.

"He was breathing and we got him in the ambulance and he was still breathing."

The chase came to an end outside the Matrix Hotel, only a few blocks from the bar, when the van rolled on its side.

Witnesses told local media they saw the suspect being pulled from the vehicle through the broken windshield and then placed in handcuffs.

The name of the suspect was not released. Knecht said he was known to police, but there was no warning for the attack.

Knecht said Edmonton police are working with the RCMP's Integrated National Security Enforcement Team and other Canadian security agencies.

"We currently believe that this individual acted alone," he said.

"The investigation is in its very early stages and we are urging Edmontonians to be vigilant and aware of their surroundings."