MOSCOW (AP) — A criminal investigation was opened after a 15-year-old boxer died following a competitive bout at a Russian youth tournament.

Anton Sidorov was knocked down twice in a fight Saturday following blows to the head at a sports club in the city of Vladimir, about 110 miles (177 kilometers) east of Moscow, the main investigative agency for the region said.

After the referee ended the fight, reportedly Sidorov’s third bout of the tournament, the young boxer began to have convulsions. He was treated by medical staff at the scene but died in an ambulance.

Investigators said in a statement that the case is being handled as a criminal matter of death caused by negligence, saying an earlier head injury could have played a role in Sidorov’s death.

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“The investigation has already established information that the child’s health required extra attention because of a serious head injury he had suffered several years before,” the regional branch of the federal Investigative Committee said.

Alexander Kiryukhin, head of the regional health department, said that Sidorov, who was a student at a local state sports academy, was not medically fit to fight. Regional governor Svetlana Orlova ordered an investigation Monday into how he was medically cleared to take part in the competition.

Authorities also said the tournament had been organized without input from the regional boxing federation.