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

Mass shootings are extremely rare in the Balkan region, although Serbia is awash in guns left over from the wars of the 1990s. No mass shootings have been reported at Serbian schools in recent years.

The shooter first killed a school guard and then three students in a hallway, according to senior police official Veselin Milic. He then entered a history classroom that was close to the school entrance and opened fire again, Milic said. Seven girls and one boy were killed, he said.

The assailant called police himself when the attack at the school in central Belgrade was over, though authorities had 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.

While Milic said the shooter planned the attack for a month, sketching classrooms and writing out a list of children he planned to “liquidate,” authorities said they did not know the motive for shooting. It was unclear if he shot any of the people whom he named on his list.

The rarity of such attacks added to the shock many felt. Commentators on television and officials repeatedly said it was the kind of thing they expected to read about elsewhere, particularly in the United States . In the last mass shooting, a Balkan war veteran in 2013 killed 13 people , including family members and neighbors, in a central Serbian village.

Authorities declared three days of nationwide mourning, starting Friday.

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

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

He 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 that the teen, who had been to shooting ranges, apparently knew the code. The father was also arrested but has yet to be charged.

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

In addition to the nine killed, six children and a teacher were also hospitalized. Two children remain in serious condition after hourslong surgeries, doctors said later Wednesday.

Ljiljana Radicevic told The Associated Press that her granddaughter was killed in the shooting. Radicevic said Ana was near the school entrance when the assailant shot the school guard, “and then he shot at my Ana. As soon as she did not answer, I knew it was over.” She did not provide Ana's full name or age.

“Today is one of the toughest days in Serbia’s modern history,” said the populist President Aleksandar Vucic, who addressed the nation in a somber manner, crying occasionally. “Unfortunately, Serbia is united in grief.”

Vucic said the shooter was taken to a special ward of a psychiatric clinic and that the police detained the teenager's mother. He didn't elaborate.

He then listed a set of proposed measures to improve gun control; tighten media and internet restrictions for violent content; and conduct drug tests in schools.

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 streets 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. Wednesday, 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 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 escaped 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.

Milan Nedeljkovic, the mayor of the Belgrade area of Vracar where the shooting happened, said most of the students were removed from the school through a back door.

“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.”

While such attacks are rare, experts have repeatedly warned of the danger posed by the number of weapons in a highly divided country, where convicted war criminals are glorified and violence against minority groups often goes unpunished.

They also note that decades of instability stemming from the conflicts of the 1990s as well as the ongoing economic hardship could trigger such outbursts.

Luka Babic, a former student at Vladislav Ribnikar, bemoaned a culture of violence.

“We can’t put the blame on this school or its teachers. ... It's a tragedy of a society that promotes violence," Babic said. "We live in the society of violence, and it has been promoted in media, in public spaces, on social media."

Education Minister Branko Ruzic, however, was quick to blame “the cancerous, pernicious influence of the internet, video games, so-called Western values." Such criticism is common among government officials in Serbia, where pro-Russian and anti-Western sentiment have increased in recent years.

Ruzic said Belgrade schools, which canceled classes after the shooting, will reopen on Thursday. Authorities have not said when classes will resume at the Vladislav Ribnikar school.