The teenage gunman who shot and killed three children and a teacher at an Oregon elementary school on Thursday opened fire at a pickup truck before storming into the school, witnesses say.

The unidentified 16-year-old was carrying a shotgun when a man in a red Toyota Tacoma pickup truck idling in Silver Rail Elementary School's parking lot noticed he was heading toward the school, The Source reported. 

Concerned by what he was seeing, the driver then decided to confront the teenager, who in turn fired numerous shots at the civilian and his truck, shattering a rear window and leaving six large bullet holes in his vehicle.

A law enforcement official briefed on the shooting told The Associated Press the teen used a pump-action shotgun that was purchased by his mother. 

Photos shared online showed the Toyota Tacoma SR5 with three bullet holes lining the bottom of the passenger-side door and another two near the front of the door. Two more bullet holes chipped off the paint on the base of the truck's bed.

The rear passenger-side window was also apparently left completely shattered, with no glass visible. 

After opening fire on the truck, the assailant allegedly threw his gun over the school fence and hopped over – prompting the driver to call Deschutes County dispatchers at around 12.59pm.

As Bend police officers then raced to the scene, the gunman shot at the school's locked doors – and was reportedly able to gain access.

It is unclear how far into the building he got before he killed three children, ages five, eight and 11, and one teacher whose age was not disclosed.

But he was then gunned down by a good Samaritan whom authorities said followed him into the building.

A seven-year-old and another 54-year-old teacher were also transported to a local hospital with critical injuries. 

Cops arrived at the scene by 1.02pm but just moments later they were informed that the lone gunman was shot and killed by an unidentified community member who had followed the teenager into the building.

When officers then entered the building, they were able to confirm that the suspect was dead.

But out of an abundance of precaution, the police officers who rushed to the scene went room by room to evacuate the building as they worked to confirm there were no additional threats to the community.

Other police officers clad in bullet-proof vests and carrying assault rifles were seen peering into the vehicles parked at the school's lot, while parents, teachers and children huddled together, embracing and wiping tears from their faces.

Bend-La Pine Schools Superintendent Dr Steven Cook held back tears as he thanked emergency responders and the school's staff for their efforts to protect students during the attack.

'Today, an unthinkable tragedy happened at Silver Rail Elementary School,' he began.

'We are heartbroken. We are horrified. This is the kind of thing you hear about in other places and pray that it never happens here... today it happened to our children, our teachers and our families.'

As Cook uttered words to his community, his voice wavered when he thanked the heroic efforts of the school staff.

'To our staff who helped our students and protected our kids today, we thank you. You did what no one should ever be asked to do. I am so grateful beyond words for your efforts,' he said.

'This is a time to love and care for one another...Please hold your loved ones tight tonight...We are going to rally and care for one another... We will get through this.'

Cook said the community was 'stunned and heartbroken by this senseless tragedy'. 

Police had asked the public to avoid the area amid the investigation, but a large crowd of concerned parents were seen gathering outside the school after hearing the reports.

Those worried parents rushed to the nearby Journey Church, to be reunited with their loved ones as officials evacuated the youngest children first.

As she waited for her nephew, Mindy Corley, the aunt of a fifth-grader at the school, told NBC News how he was left 'very scared' in the aftermath of the shooting.

She said he has a cellphone and was able to tell his mother he was OK, but 'he was clearly very, very scared and wanted to go home.' 

Martha Close, a grandmother of a 10-year-old girl at the school, also said that the reunification 'seemed to take forever,' during which time, 'I just prayed.'

Steven and Angela Nuchols also told how they got their eight-year-old daughter, Charlotte, dressed up for picture day earlier on Thursday.

They then had to race to the school just a few hours later.

'It was crawling with police officers from all over the country,' Angela said. 'You've got guys in tactical gear, rifles. You're just scared to death wondering is your child OK? You don't know.'

The couple was later reunited with their daughter, but Angela said it was 'like the worst three hours of our lives.' 

A motive for the shooting remains unclear, and authorities would not confirm on Thursday night whether the suspect knew anybody at the school or if he had attended it when he was younger.

Federal agents from the FBI's Portland Bureau and from the Bureau of Alcohol, Tobacco, Firearms and Explosives are now investigating the deadly shooting, with FBI Director Kash Patel vowing to 'make every resource [local law enforcement needs] available.'

They have already determined that the suspect used a pump-action shotgun that was purchased by his mother to commit the violent attack, a law enforcement source told the Associated Press. 

At a news conference on Thursday night, Oregon Governor Tina Kotek praised the community member she said 'heroically intervened' and told how she had visited the school nearly one year ago.

'My heart is absolutely broken today,' she said. 'No parent should have to have this kind of day.' 

She then said it was time for the community to grieve.

'There is going to be time for answering all those questions we all have right now and searching for more ways to prevent this from happening in the future,' said Kotek, a Democrat. 'But for now, let's hold each other close in grief and put as much love into the world as possible.'

The school district has now canceled all classes, activities and athletic events that were scheduled for Friday.

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