Canadian police have arrested a Somalian man suspected of stabbing an officer and deliberately ramming pedestrians during a high-speed chase in a rented truck, injuring four in what Prime Minister Justin Trudeau denounced as a “terrorist attack”.

Authorities in Edmonton confirmed the 30-year-old had applied for asylum and was known to the security services following a complaint in 2015.

“There was insufficient evidence to pursue terrorism charges,” Marlin Degrand, of the Royal Canadian Mounted Police said at a press conference, adding: “The suspect was not deemed to pose a threat to Canada.”

Police declined to identify the man because he had not yet been charged, saying the pending charges included terrorism and five counts of attempted murder.

The arrest early on Sunday came after an officer was run over outside a football game and attacked with a knife. Hours later, a U-Haul van injured four other people after swerving into pedestrians in a second attack police believe is related.

The recovery of an Islamic State flag from one of the vehicles used in the attacks had led police to pursue the incidents as an act of terrorism.

Edmonton police chief Rod Knecht said officers took a suspect into custody and they think he acted alone. The attack began outside a Canadian Football League game at Commonwealth Stadium on Saturday night. Police said a white Chevrolet Malibu rammed a traffic control barricade and sent an officer flying into the air 15ft.

Knecht said the driver then got out and assaulted 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 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 – the extent of their injuries was not immediately known. The van eventually flipped near a downtown hotel and a suspect was arrested.

“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 wre 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 did not call off the chase of the U-Haul due to the seriousness of the crime. Witness Pat Hannigan told reporters he saw police pull the man from the windshield of the toppled U-Haul. He estimated 30 police cars were chasing the U-Haul.

Austin Elgie, manager of the Pint bar just west of the downtown core, also saw the van zoom by with police giving chase. The van “peeled” into an alley where people were smoking, he said.

“It was crazy,” he said. “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 said. “He was breathing and we got him in the ambulance and he was still breathing.”

Rachel Notley, the Alberta premier, said on Twitter her thoughts were with the injured officer and she was hoping for a speedy recovery.

It was military appreciation night at the football game between the Edmonton Eskimos and the Winnipeg Blue Bombers. Canada’s chief of defence staff, Gen Jonathan Vance, conducted the pregame coin flip at Commonwealth Stadium 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.