The teenager accused of killing nine people in Belgrade on Wednesday had a detailed plan, including a kill list, that was pulled together over a month, police said.
The shooting occurred at 8:40 am (0640 GMT) at an elementary school in Belgrade's downtown Vracar district.
The suspect "planned the shooting for a month and made a list of kids he planned to kill," Veselin Milic, Belgrade's police chief, told a press conference.
"The sketch looks like something from a video game or a horror movie, which indicates that he planned in detail, by classes, whom to liquidate," Milic added.
The incident rocked the Balkan nation, where there has not been a school shooting of this magnitude in decades.
Police moved quickly to seal off the neighbourhood as parents rushed to the scene, where students were visibly distraught as they waited outside the school.
"Eight children and a security guard were killed, while six children and one schoolteacher were wounded," the interior ministry said earlier.
"All police forces are still on the ground and are intensively working to shed light on all the facts and circumstances that led to this tragedy."
Gun violence in schools is extremely rare in Serbia, where purchasing a firearm requires a special permit.
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