Belgrade, Serbia - A young teenage boy went on a shooting spree in his own school in the Serbian capital of Belgrade on Wednesday morning, opening fire in his classroom with a handgun and killing nine people.

Eight schoolchildren and a security guard were killed at the school, Veselin Milic, director of the Belgrade police, said at a press conference in the Serbian capital on Wednesday afternoon. Six other kids and a teacher were wounded, including a girl who suffered life-threatening injuries.

Milic said the suspected shooter, a student at the school who was born in 2009, had premeditated the attack and even had a list of potential victims. The boy was arrested in the schoolyard.

The government in Belgrade ordered three days of national mourning.

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There was initially no information about the exact motives for the shooting at the Vladislav Ribnikar primary school in the Vračar district of Belgrade.

But Milic said the boy appears to have been planning his crime for months. Sketches and plans were found on his desk that looked "like something out of a horror film or video game," Milic said.

The boy went to his school with his father's gun and four incendiary devices in his pocket, where he first killed a security guard and two students, Milic said.

Then he went to his school class, where history class was in progress. He loaded his gun and opened fire from the doorframe, at the teacher and several students, according to the police chief.

He then ran into the schoolyard and called the police, Milic said.

Belgrade police blocked off a wide area around the school, as a large contingent of officers and rescue workers was deployed to the building.

Education Minister Branko Ruzic and Health Minister Danica Grujicic also visited the scene.