The parents of a 13-year-old accused of killing 10 people in a school shooting in Serbia last year have gone on trial for allegedly failing to safeguard the weapon and ammunition used in the attack. The massacre last May – and a second mass shooting a day later – rocked the Balkan nation, setting off major anti-government demonstrations that led to the formation of an opposition coalition that stood in recent elections. The teenager, now in a mental hospital, allegedly shot nine of his classmates and a security guard at his Belgrade school with his father’s weapons. The boy was 13 at the time of the attack, making him not criminally liable under Serbian law. Prosecutors say the father had trained the boy to shoot, did not properly secure his weapons and ammunition, and allowed his son to hide a handgun and 92 bullets in his backpack that he later used in the shooting. He is also charged with a “serious act against general safety”, while the mother is accused of illegal possession of ammunition. “I expect a legal and fair trial, at the end of which the court will convict the defendants of the criminal offences against them,” the chief prosecutor, Nenad Stefanovic, told the Serbian broadcaster RTS on Monday. Both parents pleaded not guilty to the charges, according to the broadcaster. Prosecutors have also charged the head of a Serbian shooting club and an instructor with providing false testimony. The trial will be held behind closed doors, the court announced on Monday, preventing the press and members of the public from attending the proceedings. The decision was denounced by Irina Borovic, the lawyer representing the parents. “We hope that the court will reconsider this decision during the trial and that the public will be informed,” Borovic told local media. As the trial began, the 21-year-old suspect in the second mass shooting was formally charged with murdering nine people and injuring 14 others on a rampage across three villages. The 23 victims were gunned down with an automatic weapon in a series of drive-by shootings around the village of Mladenovac, about 60km (37 miles) south of the capital. RTS reported that the suspected gunman also faced charges of kidnapping and illegal possession of weapons. Serbia has one of the highest gun ownership rates in the world, with 39 firearms for every 100 civilians, according to the Small Arms Survey project. After the shootings, the Serbian president, Aleksandar Vučić, vowed to “disarm” the nation with an ambitious plan that would crack down on both legal and illicit firearms. Despite the pledge, the shootings led to huge anti-government protests as tens of thousands called for the resignation of top officials and an end to the glorification of violence and gangster culture in the media. Vučić largely dismissed the protests as a “political” stunt, and peddled conspiracy theories about foreign powers allegedly orchestrating the rallies.