“I was able to hear the shooting. It was non-stop,” a student who was in a sports class downstairs when the gunfire erupted. “I didn’t know what was happening. We were receiving some messages on the phone.”

Unlike in the United States, mass shootings in Serbia and in the wider Balkan region are extremely rare; 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 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.

Local media footage 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.

The student who heard the shooting, who was identified only by her initials, E.M., because of her age, described the suspect as a “quiet guy” who “looked nice.”

“He was having good grades, but we didn’t know much about him,” the student added. “He was not so open with everybody. Surely i wasn’t expecting this to happen. ”

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 school, in the center of Belgrade.