Authorities are investigating a serious assault after a man attack a school for elementary children.

Twenty children were wounded in an attack at an elementary school in Beijing on Tuesday with three suffering serious injuries, local authorities said.

A male suspect was brought “under control” and the wounded students were admitted to hospital, the Xicheng district government said, without elaborating on how or why the man carried out the attack.

According to a tweet by nationalist tabloid Global Times, the suspect used a hammer to carry out the assault.

Beijing police did not immediately respond to AFP’s request for comment.

Violent attacks targeting schoolchildren are not uncommon in China, which has seen a slew of deadly incidents over the past few years.