The 13-year-old who opened fire on Wednesday at his school in Serbia’s capital drew sketches of classrooms and made a list of children he intended to target in a meticulously planned attack, police said.

The shooter killed a school guard and then three students in a hallway of the Vladislav Ribnikar school in central Belgrade, according to senior police official Veselin Milic. He then entered a history classroom close to the school entrance and opened fire again, Milic said. Seven girls and one boy were killed, while six children and a teacher were hospitalised.