Police received a report of the shooting at Belgrade’s Vladislav Ribnikar elementary school Wednesday morning. Officers dispatched to the scene detained the boy in the schoolyard, the statement said.

In Photos: The scene after 8 children and 1 guard were killed in a school shooting in Serbia

The boy burst into a classroom, opening fire at the teacher first and then at students, who hid under their desks, one parent told Serbian broadcaster N1. The school’s security guard was shot and killed while trying to stop the rampage. Another student lay on top of her friends’ bodies, pretending to be dead to avoid getting shot, she recounted on Serbian television.

Authorities said they were investigating the suspected shooter’s motive. Schools in the capital closed and announced three days of national mourning. Parents gathered outside the school, where ambulances were stationed and police blocked off roads.

The boy cannot be held criminally responsible since he is under the age of 14, according to the public prosecutor’s office in Belgrade, Serbian broadcaster RTS reported. The boy’s father was also taken into custody and faces two to 12 years in prison. Police told Serbian media the boy had the code to the safe where his father’s weapons were kept.

Belgrade police official Veselin Milic said the student had drawn a sketch of classrooms and wrote a list of classmates he planned to attack, Serbian media reported. He said the shooter also called police himself when the rampage was over. Seven girls and one boy were killed, Milic said — in addition to the security guard.

In a statement Wednesday, Belgrade Mayor Alseksandar Sapic said the school shooting was “unprecedented in the history of our city,” expressing his condolences to the families of the children and guard who were killed.

The head of a hospital treating the victims said the injured children sustained gunshot wounds to their legs and arms, while the teacher was shot in the stomach. Two of the students and the teacher were stable and conscious, he said in an update later Wednesday.

“This tragic incident serves as a reminder of the importance of mental health awareness and support, particularly in the education system,” the U.N. children’s agency UNICEF said in a statement.

U.S. Ambassador to Serbia Christopher Hill said on Twitter Wednesday he was “saddened and in shock.”

“One child’s death is a tragedy; for so many to be murdered where they should feel safest is unbearable,” he said. “America and the world share your grief.”

Mass shootings are relatively rare in Serbia, even though illegal guns are widespread in the Balkans — a remnant of the wars of the 1990s.

The last mass shooting in the country was nearly a decade ago, when a veteran killed 13 people in 2013 in a central Serbian village, according to the Associated Press.