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.

A father of a student said the shooter entered his daughter’s classroom, firing at her teacher and then her classmates as they ducked under their desks. Most students at the school were able to flee through a back door, according to a local official.

Authorities declared three days of nationwide mourning, starting Friday.

Mass shootings are extremely rare in Serbia and in the wider Balkan region; none were reported at schools in recent years. In the last mass shooting, a Balkan war veteran in 2013 killed 13 people in a central Serbian village.

Experts, however, have repeatedly warned of the danger posed by the large number of weapons in the country after the wars of the 1990s. They also note that decades of instability stemming from the conflicts as well as the ongoing economic hardship could trigger such outbursts.

Police identified the shooter as Kosta Kecmanovic, who attended the Vladislav Ribnikar school, where students would typically range in age from 6 to 15.

Attacker Carried Guns and Explosives

Kecmanovic carried two guns belonging to his father — at least one a handgun —and four Molotov cocktails, officials said. Interior Minister Bratislav Gasic said the weapons were licensed and kept in a safe but the teen, who had been to shooting ranges, apparently knew the code. The father was also arrested.

It’s not clear how many rounds were fired, but police said the shooter reloaded the handgun at one point.

Police showed reporters a sketch they said he had drawn of classrooms and Milic said he also wrote out a list of children he planned to “liquidate” in the attack that he planned for a month.

In addition to the nine killed, six children and a teacher were also hospitalized.

Local media footage showed a commotion as police removed Kecmanovic, whose head was covered as officers led him to a car. Police sealed off the blocks around the school. Authorities later carried body bags to a waiting van.

Police said they received a call about the shooting at around 8:40 a.m. on the first day that classes resumed after a long weekend for the May 1 holiday.

“I was able to hear the shooting. It was nonstop,” said a student who was in a sports class when gunfire erupted elsewhere in the building. Her mother asked that her name be withheld because of her age. “I didn’t know what was happening. We were receiving some messages on the phone.”

The student described the shooter as a “quiet guy” who had good grades.

“He was not so open with everybody. Surely I wasn’t expecting this to happen,” she said.

Milan Nedeljkovic, the mayor of the Belgrade area of Vracar where the shooting happened, said that most of the students were taken out a back door of the school.

“We have video surveillance, but now this is a lesson, we need metal detectors too,” he said. “It is a huge tragedy … something like this (happening) in Belgrade. Such a tragedy at an elementary school.”

Four students and a teacher were sent to University hospital, according to the hospital’s director, who said one child and the teacher were in serious condition.

Milan Milosevic, who said his daughter was in a history class when the shooting took place, told N1 television that he rushed to the school when he heard what had happened. He received a call from his daughter who had gotten out of the building and was unharmed.

“He (the shooter) fired first at the teacher and then the children who ducked under the desks,” Milosevic said his daughter told him.