Mr. Zivkovic said he heard the gunshots from his school Wednesday morning and initially mistook them for the ignition of a truck. He then realized, after hearing the wail of police and ambulance sirens echo through the neighborhood, that a shooting had taken place.

“This is madness,” he said.

The shooting occurred on Wednesday morning, when the boy, who was in seventh grade, opened fire. Six children and a teacher were also injured in the attack and taken to the hospital. One girl who was wounded underwent emergency surgery overnight and was in critical condition Thursday, officials said. Among those killed was a French girl, according to the French Foreign Ministry.

Serbian officials revealed the identity of the boy at a briefing Wednesday, but The New York Times is withholding his name since he is a minor and has not been charged. The attack sent shock waves through the country, which has experienced few mass shootings, let alone violence of this sort at a school. “Nobody thought this could happen here in this neighborhood, here in Serbia,” said Anita Lainovic, 45, who had come to the school with a pot of flowers to pay homage to the young victims.

Many residents wondered what might have driven the student to kill his classmates in what the police described as a planned, coldblooded attack. The student entered the school with two pistols belonging to his father and four Molotov cocktails, which he had prepared himself.

The student had accompanied his father to a shooting range in the past and knew how to shoot, Serbian officials said.