BELGRADE, Serbia (AP) — A trial started Monday in Serbia for the parents of a teenager who is accused of killing 10 people and injuring six in a mass shooting at his school last May that left the Balkan nation in shock.

The suspected shooter, 13-year-old Kosta Kecmanovic, has been held in a mental institution since the attack and cannot be held criminally liable under Serbian law because of his age. His father and mother were charged with a “serious act against general safety” for failing to safeguard the weapon and ammunition used in the shooting.

The High Court in the capital, Belgrade, decided to keep the entire proceedings closed to the public despite calls by the defense lawyers that they be open. The couple, reportedly embraced in the courtroom and wept together, according to local media reports.

The shooting at a school in Belgrade last May 4, which left nine schoolmates and a security guard dead, was followed by another mass slaying a day later in central Serbia that killed eight people and wounded 14. The two attacks triggered months of protests against Serbia’s populist President Aleksandar Vucic for allegedly creating a culture of violence in a country that went through a series of bloody wars in the 1990s.

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Kecmanovic’s father faces additional charges, including an accusation of training the boy how to shoot without properly guarding the weapons at their home. The manager of a shooting range and an instructor also have been charged.

Serbia has one of the highest gun ownership rates in the world. The country is full of weapons left over from the conflicts of the 1990s.

Chief prosecutor Nenad Stefanovic told state RTS broadcaster that he expects “a free and fair trial.”

The defense lawyers said Monday they are against keeping the trial closed to the public.

“Today the court made a decision to exclude the public in the entire course of this procedure, stating that this is done to protect the interests of minors and some private interests of the participants in the procedure,” lawyer Irina Borovic said. “Our position is that the decision of the court was absolutely hasty.”