At least 11 people were killed and 13 others injured in eastern China early Tuesday when a bus crashed into a group of students and their parents at a school.

The students and parents were at the gate of a middle school in Tai’an city in the eastern province of Shandong just before 7:30 a.m., the Dongping county police department said in a social media post.

Six parents and five students were killed, it said. One of the injured was in serious condition while the others were listed as stable, the department said.

The driver was in police custody and the incident was under investigation, it said.

The bus was specially customized for transporting students, it said. It did not say who was responsible for operating the bus. Many schools contract out such services to private companies or individuals.

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.