A teenage gunman killed three children and a teacher when he opened fire at an elementary school in Oregon on Thursday afternoon.

The unidentified shooter, 16, shot at the locked doors of Silver Rail Elementary School in the city of Bend on Thursday afternoon before killing three children, ages five, eight and 11, and one teacher whose age was not disclosed.

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. 

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. 

'Today is an awful day for our community,' Bend Mayor Melanie Kebler declared. 'Our hearts are broken as we mourn this tragedy and the profound impact it has had on so many people, especially the children, families, educators and school staff whose lives have been forever changed.

'A school should be a safe place where children can learn and feel secure,' Kebler continued, as she called for the community to lean on one another.

'We are here for you. We grieve alongside you and we will get through this together.' 

The tragedy began when a man driving a red Toyota Tacoma pickup truck in the school parking lot spotted the teen carrying a long gun approach the building, according to The Source. 

The driver then confronted 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 the vehicle.

At that point, 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.

Within just one minute, the Bend police department sent out an alert to respond to a critical scene. 

But as officers then raced to the school, the suspect forced his way into the building by shooting at locked doors - at which point dispatchers started receiving 'dozens' of 911 calls, Police Chief Mike Krantz said at a news conference Thursday night.

Cops arrived at the scene by 1.02pm and just moments later, the lone shooter was killed by an unidentified community member.

The school was placed under lockdown following the shooting, as police officers went room by room to evacuate the building, Miller said. The school has a student body of between 400 to 450 pupils and a staff of more than 40.

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

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 have since 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.' 

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.' 

Meanwhile, Federal agents from the FBI's Portland Bureau and agents from the Bureau of Alcohol, Tobacco, Firearms and Explosives have descended on the scene to aid in the investigation, with FBI Director Kash Patel vowing to 'make every resource [local law enforcement needs] available.'

It has since been determined that the suspect used a pump-action shotgun that was purchased by his mother, a law enforcement source told the Associated Press. 

At a press 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.'

Bend-La Pine Schools Superintendent Steven Cook also called the incident on Thursday 'a senseless tragedy.'

'No community should have to do what we are doing right now,' he said at the news conference, visibly shaken by what had happened. 'I don't have any words for what this community is feeling right now.'

Police spokeswoman Sheila Miller had made a similar remark earlier in the day.

'This is the kind of thing we never want to see happen in Bend,' police spokeswoman she said, choking up as she spoke.

'And our hearts go out to this community, and we ask our community to really work together and come together at this time.' 

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

Following the tragedy, Congresswoman Janelle Bynum also announced that she is halting all campaign activity, saying she is 'focused on supporting and being with the community as it responds to this tragedy.

'I feel this deep, deep, deep in my soul,' she said in a statement. 'Our whole community does. Our whole state does. And the whole nation should.

'Our babies are supposed to do arts and crafts at school, not use their desks as cover from gunfire. They're supposed to make friends, not mourn them. This is every parent's worst fear.

'My heart and my prayers and everything else I got is with the families at Silver Rail Elementary. When will it end?'

Senator Ron Wyden also said: 'My heart is with the victims of today's shooting at Silver Rail Elementary School and their loved ones now facing the consequences of another school shooting in America.'

'Once again we must ask how many schools must be shot up before Republicans agree to act to curb gun violence,' he continued.