A man with a hammer attacked children at a primary school in Beijing Tuesday, injuring 20, police said.

Three of the victims were “badly injured” — though authorities said their wounds were not life-threatening.

Police have arrested a suspect, but said a motive was not yet known.

The attack took place at the No.1 Affiliated Elementary School of Beijing Xuanwu Normal School, in central Beijing’s Xicheng district, according to a local police Weibo account.