A teacher threw her mobile phone at a teenage gunman before tackling him to the floor of an Oregon elementary school classroom, in an act of courage that police say almost certainly prevented an even greater loss of life.

Three children and a teacher were killed in Thursday's attack at Silver Rail Elementary School in Bend. The 16-year-old attacker, a former pupil, was shot dead by an armed father of two pupils who had been driving past and saw the shooting begin.

Police identified two of the dead on Friday as five-year-old Rhoam Summer and 25-year-old teacher Meghan Lambert. The families of the two other children killed, boys aged eight and 11, asked that their names not yet be made public.

A struggle in the classroom

Bend Police Chief Mike Krantz said the attacker scaled a fence around the school moments before the shooting, after firing at a vehicle in the car park and throwing his weapon over the fence before retrieving it.

All of the victims were in a special educational needs classroom. Police said the teenager fired as many as 15 rounds from a pump-action shotgun and was carrying about 44 more rounds when he was stopped.

A female teacher threw her phone at the gunman, then grappled with him and tried to wrestle the shotgun from his hands as they struggled on the floor. She was not injured.

"She was not injured in that incident but she almost certainly helped prevent more bloodshed," Chief Krantz said, praising both the teacher and the father who intervened as heroic and selfless.

The father, who has two daughters at the school, had been driving past at lunchtime when he saw the teenager firing in the car park and then shooting through a window to get inside. He ran into the building carrying a pistol and shot the attacker multiple times.

"To the people who showed courage in the face of such violence, and intervened to prevent this tragedy from being any worse, we owe you a debt of gratitude," Krantz told a news conference on Friday.

The victims

Two others were injured in the attack. A 54-year-old teacher remained in hospital on Friday, and a seven-year-old pupil was described as being in a serious but stable condition.

The family of Rhoam Summer, the youngest victim, said in a statement that his death had left them "broken and lost". Teacher Meghan Lambert was also a volleyball coach at the school.

Bend-La Pine Schools superintendent Steven Cook confirmed the attacker had previously been a special needs student at Silver Rail but was not currently enrolled in the district.

A makeshift memorial of flowers, candles and soft toys grew outside the school on Friday as investigators worked to establish how the teenager obtained the weapon and what lay behind the attack.

A familiar American grief

The shooting is the latest in a long line of attacks on American schools, and it has already renewed debate about gun access and school security. Police said the investigation into how the 16-year-old obtained the shotgun was continuing.

Officers said the entire attack unfolded over a matter of minutes. The first emergency call came from a person in a vehicle at 12:59 local time on Thursday, reporting gunfire at the school.

Community members gathered near the school through Friday, leaving tributes and comforting one another. Local churches opened their doors for vigils, and counsellors were made available to pupils, families and staff across the district.

For Bend, a city of around 100,000 people in central Oregon better known for its outdoor life than for violence, the attack has come as a profound shock. City leaders said schools in the area would remain closed while the investigation continues and support is put in place.

Police have not suggested any wider threat to the public. The focus now, Chief Krantz said, is on supporting the families of the dead and injured, and on understanding how a former pupil came to return to his old school with a loaded shotgun.