The man who attacked 20 Beijing schoolchildren with a hammer Tuesday was a frustrated maintenance worker at the school, local authorities said.

The 49-year-old man’s labor contract with the school was set to expire this month, according to the local government in Xicheng, the central Beijing district where the attack took place.

On Tuesday, the man used a hammer to attack students at the No. 1 Affiliated Elementary School of Beijing Xuanwu Normal School. He injured 20 students, several of whom required intensive care, said Li Jia, deputy secretary of the party committee at Xuanwu Hospital. No deaths occurred.

Wang Shaofeng, head of the Xicheng government, didn’t directly connect the attack with the man’s work contract, but said the man was “unsatisfied” and had “vented.” According to Wang, prior to the attack, the school was in talks to help arrange a new job for the man.

A school spokesperson denied any employment dispute with the man.

At a press conference after the attack, Liu Yuhui, director of the Beijing Municipal Education Commission, urged education institutions to “activate internal safety checks,” including crew inspections. District head Wang bowed to apologize for the tragedy.

Related: Man Attacks Beijing Primary School Students With Hammer