Belgrade, Serbia —  A 13-year-old who opened fire Wednesday at his school in Serbia's capital drew sketches of classrooms and wrote a list of people he intended to target in a meticulously planned attack, police said. He killed eight fellow students and a school guard before being arrested.

The shooter first killed a guard at the school in central Belgrade and then three students in a hallway, according to senior police official Veselin Milic. He then entered a history classroom — apparently choosing it simply because it was close to the entrance — and opened fire again, Milic said.

The assailant called police himself when the attack was over, though authorities received a call reporting the shooting two minutes earlier.

The suspect was a seventh grade student at the school, police said. The statement added that the boy apparently used a gun belonging to his father.

Reports said terrified parents arrived to the school trying to find their children. Local media video from the scene showed commotion outside the school as police removed the suspect, whose head was covered as officers led him to a car parked in the street.

Mass shootings in Serbia and in the wider Balkan region are extremely rare and none has been reported in schools in recent years. 

By comparison, almost 250 people have been killed in mass shootings in the U.S. already this year, including 12 in the last week of April. Recent research found that fatalities from gun violence in the U.S. have increased over time, with more victims dying at the scene of shootings before they can be transferred for medical treatment.    

In the last mass shooting in Serbia, a Balkan war veteran killed 13 people in 2013 in a central Serbian village. 

Experts have repeatedly warned of the number of weapons left over in the country after the wars of the 1990s. They also note that decades-long instability stemming from the conflicts as well as the ongoing economic hardship could trigger such outbursts.

Milan Milosevic, who said his daughter was in a history class when the shooting took place, told N1 television that he rushed out when he heard what had happened.

"I asked, 'where is my child,' but no one could tell me anything at first," he said. "Then she called, and we found out she was out."

"He (the shooter) fired first at the teacher and then the children who ducked under the desks," Milosevic quoted his daughter as saying. "She said he was a quiet boy and a good student."

Police sealed off the blocks around the Vladislav Ribnikar school, in the center of Belgrade. Primary schools in Serbia have eight grades.

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• Students Killed
• Shooting
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• Serbia