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.

A suspect, who has not been identified, is in custody, said Chief Rod Knecht of the Edmonton Police Service in an early Sunday news conference.

Knecht said authorities believe the man acted alone, but haven't ruled out the possibility that others might be involved.