Some readers may find this article distressing.

A female teacher in the US state of Oregon tackled a teenage shooter who entered her classroom and tried to wrestle the gun from his grasp, police have said.

The teacher, who has not been named, threw her mobile phone before trying to disarm the 16-year-old on the floor of a classroom at Silver Rail Elementary in the city of Bend.

The attacker killed three children and a teacher after shooting his way into the building on Thursday.

He was shot dead by a father with two daughters at the school, who was driving past as the attack started to unfold and, armed with a pistol, ran in to confront the suspect.

Police said the attacker used to be a student at the school. His motive remains unclear.

He had been armed with about 44 more rounds of shotgun ammunition when he was stopped, police said. He fired as many as 15 rounds in the class.

"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," Bend Police Chief Mike Krantz said.

Police used a news conference on Friday to identify two of the four victims of the school shooting. They were five-year-old Rhoam Summer and 25-year-old teacher Meghan Lambert.

Lambert's family said in a statement shared by the police that her goal was to be a teacher in the district in which she grew up and that she was "compassionate and had a huge heart".

"She always fought for the underdog and those with special needs," they said.

Bryan Jolly, Summer's grandfather, said the family was devastated by his death.

"The whole thing is senseless, and a shock," he told the New York Times, saying his grandson was "was very bright and very mischievous and very fun-loving, and totally without fear".

The families of the two other students who were killed, an eight-year-old and 11-year-old, did not wish to have them publicly identified yet, police said.

Two others were injured in the attack. Another teacher, a 54-year-old, was still being treated at hospital, and a seven-year-old student, who was in serious but stable condition.

Krantz said the heroic actions of both the teacher and the father who ran inside the school had saved lives.

He said the teacher who wrestled with the gunman had not been hurt but "almost certainly helped prevent more bloodshed".

Krantz identified the suspect as 16-year-old Griffin J John II, who had been a special educational needs student at the school.

A search warrant has been executed at his home and a search is ongoing, the police chief said.

Police believe the suspect travelled to the elementary school on foot, but they are still investigating his exact movements and how he acquired the gun.

Officials did not confirm the name of the father who intervened and fatally shot the suspect.

But Kaylub Giamanco's family has identified him to US media as the person who sprang into action.

The 36-year-old carpenter had just finished lunch and was on his way home when he passed the school, his brother told NBC News. He saw the suspect open fire on a vehicle outside the school and then shoot through a window to gain entry.

Police said he had a legally concealed .40 calibre in his vehicle, while his brother said he grabbed the pistol "and decided to go in after" the suspect. Police say he fired at the suspect "multiple times", killing him.

Giamanco's mother, Kenda Freeman, who lives in Sacramento, California, told a local CBS station that her son had called her moments after the shooting and that he was "traumatised".

"He said 'a man went into my daughter's school'," Freeman recalled, pausing as she started to tear up. "And, um, shot kids. And he said, 'and I looked at him point blank, and I shot and killed him, mum'."

She added: "I told him he saved a lot of lives, and he said, 'mum, I couldn't save them all'."

US President Donald Trump praised the bystander on Friday, calling him a "great hero" and telling reporters that the situation could have become "much worse".

"It's a terrible thing, but thank goodness for a man that had a gun and knew how to use it," he said.

Without naming the bystander, Oregon Governor Tina Kotek acknowledged the man's heroic actions as having stopped further loss of life, but noted they "carry a heavy emotional burden and I just want you to know that my thoughts are with you as well".

Additional reporting by Kayla Epstein