BEIJING (AP) — A bus crashed into a group of students in eastern China on Tuesday, killing or injuring at least 10, state media reported.

The students were waiting at the gate of a middle school in Tai’an city in Shandong province shortly after 7 a.m., Xinhua news agency said.

The bus was specially customized for transporting students, it said.

The cause of the accident was being investigated. School safety, including overloaded school buses and poorly designed buildings, has long been a problem in China.

In 2017, a dozen people, including 11 kindergarten pupils, were killed when a school bus crashed and burst into flames in a tunnel in the eastern Chinese city of Weihai, also in Shandong province. The driver, six Chinese children and five South Korean children were killed. It remains unclear whether the crash was deliberate or the result of unsafe driving.

China has cracked down heavily on transportation dangers, adding training and vehicle inspections.

China also has suffered numerous cases in recent years of attacks on school children, often using knives or homemade explosives. The suspects were generally found to be bearing grudges and seeking revenge over personal matters or against society more generally.