A 16-year-old shot his way into an Oregon elementary school. An armed bystander ended it in minutes.

A 16-year-old gunman killed three children and a teacher at an elementary school in Bend, Oregon, on Thursday afternoon before an armed community member chased him into the building and shot him dead, according to Bend police and the Associated Press.

The attack at Silver Rail Elementary — a school of about 460 pupils from kindergarten to Grade 5, roughly 200 kilometres southeast of Portland — unfolded around 1 p.m. local time, just over an hour before dismissal, the AP reported.

Bend Police Chief Mike Krantz said the first 911 call came at 12:59 p.m. from a person in a vehicle outside the school who reported being shot at multiple times by someone with a long gun. The teen fired at the school's locked entrance doors and forced his way inside, the chief told reporters at a Thursday night news conference. Officers arrived about three minutes later. By then, the suspect was already down.

"An officer reported that the suspect had been shot by a community member who had seen the suspect enter the school and pursued him into the school and engaged him," Krantz said, according to the AP.

The children killed were aged 5, 8 and 11, police confirmed. A 7-year-old student and a 54-year-old teacher remained in hospital in critical condition on Friday. Police have not publicly released the victims' names, saying families were still being notified, and have not publicly identified the suspect or the person who stopped him.

The weapon was a pump-action shotgun purchased by the 16-year-old suspect's mother, a law enforcement official briefed on the investigation told the AP, speaking on condition of anonymity because they were not authorized to discuss the case publicly. Investigators have not established a motive or whether the teen — described by police as a Bend resident — had any connection to the school. The FBI is assisting with the investigation.

Oregon Gov. Tina Kotek thanked the unidentified civilian at a news conference. "To the community member who heroically intervened to stop further loss of life: Your actions carry a heavy emotional burden," she said, adding that her thoughts were with him as well. The AP reported Friday that the man is the father of two girls who attend the school, according to a statement from his mother read to reporters; the family asked for privacy.

Bend-La Pine Schools superintendent Steven Cook said the district's "immediate priority is to care for our students and staff and to support law enforcement and emergency responders." The district cancelled all classes and activities for Friday and said crisis counsellors would be available.

For Canadians, the attack lands in a familiar and painful debate. Canadian schools run lockdown drills for exactly this scenario, and educators and parents across the country will be watching how Bend's investigation answers the same questions that follow every such attack: how the gunman got his weapon, and how he got through locked doors.