Police said eight children and a security guard were killed when a teenage boy opened fire at a school in Serbia's capital of Belgrade on Wednesday.

At least six additional children and one teacher were also injured and are receiving emergency treatment.

Police said the 13-year-old assailant  "planned the shooting for a month and made a list of kids he planned to kill."

He used two of his father's guns in the shooting at the Vladislav Ribnikar elementary school, whose students would typically range in age from 6 to 15. He was also apparently armed with petrol bombs. 

Interior Minister Bratislav Gasic said the gun was kept in a safe but the teen apparently knew the code. He said the father was also arrested.

The boy, who was earlier described as 14-year-old, can't face criminal charges because of his age, the Belgrade prosecutor's office said. Social Services will determine what happens to him.

Suspect had 'detailed' plan

Police said they responded to a call about the shooting around 8:40 a.m. local time. The shooter himself was among the people who called police about the attack.

According to police, the teenager had a sketch of classrooms and also wrote out a list of children he planned to "liquidate." 

"The sketch looks like something from a video game or a horror movie, which indicates that he planned in detail, by classes, whom to liquidate," Veselin Milic, Belgrade's police chief, said. 

Police were investigating a motive. "All police forces are still on the ground and are intensively working to shed light on all the facts and circumstances that led to this tragedy," the interior ministry added.

Addressing the nation following the attack, Serbian President Aleksandar Vucic said it was "one of the most difficult days in the modern history of our country."

He said the suspected shooter would be held in custody in a "psychiatric ward."

Vucic also proposed implementing a two-year moratorium on issuing firearms permits in response to the mass shooting.

How the attack unfolded

Authorities said the shooter first killed a guard and then three students in a hallway.

He then entered a classroom and opened fire again. 

"I was able to hear the shooting. It was non-stop," a student who was in a sports class downstairs when the gunfire erupted. "I didn't know what was happening. We were receiving some messages on the phone."

One of the children who witnessed the shooting told her father she was in the classroom when the shooting began.

"She managed to escape. (The boy) ...first shot the teacher and then he started shooting randomly," the girl's father, Milan Milosevic, told broadcaster N1.

Milosevic rushed to the school when the shooting was first reported. "I saw the security guard lying under the table. I saw two girls with blood on their shirts. They say he (the shooter) was quiet and a good pupil. He recently joined their class," he said.

Most students were able to flee through a back door, according to a local official.

Mass shootings not common in the Balkans

Mass shootings are rare in Serbia and purchasing a firearm requires a special permit.

However, many firearms left over from the wars of the 1990s are still in circulation.

The last mass shooting in Serbia happened in 2016 when a man killed five people and wounded twenty-two with an assault rifle in a cafe in the town of Zitiste.

Because the school shooter is under 14, Kecmanovic can't face criminal charges, the Belgrade prosecutor's office said. Social services will determine what happens to him.

lo/sms (AP, AFP, Reuters)