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(CNN) — Canadian authorities say two incidents in Edmonton are being investigated as acts of terrorism after a man struck a police officer with a car before stabbing him and later plowed a truck into pedestrians on a busy street, injuring at least four people.

In a Sunday afternoon press conference, authorities said the suspect is a male Somali national and refugee who was previously known to Edmonton police and the Royal Canadian Mounted Police (RCMP).

Police have arrested the suspect for several offenses, including participation in a terrorist act and the commission of an offense for a terrorist group, said Chief Rod Knecht of the Edmonton Police Service.

The suspect was investigated in 2015 after police received a complaint he was “espousing extremist ideology,” said RCMP Deputy Criminal Operations Officer Marlin Degrand, but there was insufficient evidence to pursue terrorism charges. The suspect was not deemed to be a threat at the time, Degrand said.

Formal charges have not been filed against the suspect, who police believe acted alone. Police had not released his name as of late Sunday afternoon.

The first incident happened Saturday at 8:15 p.m. (10:15 p.m. ET) when the driver of a white Chevrolet Malibu drove it into a traffic barricade. Police had set up the roadblock near Commonwealth Stadium for an Edmonton Eskimos football game.

A man drove the car at high speed, striking the police officer who had been standing between a traffic barricade and his cruiser.

“The vehicle struck the police officer, sending him flying into the air 15 feet, before colliding with the police officer’s cruiser again at a high rate of speed,” Knecht said in a Sunday morning news conference.

The driver jumped out of the vehicle, attacked and stabbed the officer several times with a knife and fled on foot, Knecht said.

There was an ISIS flag in the car, which has been seized as evidence, the police chief said.

Knecht said Sunday afternoon that the wounded officer, Mike Chernyk, had severe abrasions on his arms and suffered stab wounds to his face and head but will make a full recovery.

After that incident, police broadcast the name of the Chevrolet’s registered owner to patrol officers in the city.

Just before midnight, a police officer stopped a U-Haul truck at a checkpoint and recognized the driver’s name as similar to that of the Chevrolet’s registered owner. The U-Haul truck then sped off towards downtown Edmonton with police chasing it.

The U-Haul traveled on busy Jasper Avenue, which late Saturday night and early Sunday morning was full of football fans and bargoers, reported CNN partner CBC. During the chase, the truck deliberately attempted to hit pedestrians in crosswalks and alleys in two areas along Jasper Avenue, Knecht said.

At least four pedestrians were injured. By Sunday afternoon, two of the victims had been released from the hospital, Knecht said. The others were kept for further observation.

As police gave chase, Knecht said, there was no indication the suspect would stop.

“As a result, a deliberate tactical maneuver by a police vehicle was authorized and successfully executed, causing the suspect vehicle to roll onto its side,” Knecht said.

Officers then broke the truck’s windshield and used a stun grenade on the suspect before attempting to remove him. When he refused, they used a Taser on him and arrested him, Knecht said.

“No shots were fired,” Knecht said. “In fact, no shots were fired anywhere in this entire incident.”

Formal charges are expected to be filed “in the near future,” Degrand said.

In a press conference earlier Sunday, Edmonton Mayor Don Iveson urged residents to remain calm.

“To the best of our knowledge,” he told reporters, “this was a lone wolf attack.”

“The Government of Canada and Canadians stand with the people of Edmonton after the terrorist attack on Saturday that sent an Edmonton Police Service officer to (the) hospital and injured a number of innocent people who were out to cheer on their football team and to enjoy an evening in their city,” said Canadian Prime Minister Justin Trudeau in a statement Sunday.

“We cannot — and will not — let violent extremism take root in our communities,” he added. “We know that Canada’s strength comes from our diversity, and we will not be cowed by those who seek to divide us or promote fear.”