BEIJING (AP) — Authorities say 21 people were killed Tuesday when a bus ran through a roadside fence and plunged into a lake in a southwestern Chinese city.

Surveillance video posted by state broadcaster CCTV on its social media account show the bus suddenly race across six lanes of traffic and through the fence. The cause of the accident was unclear.

The Anshun city government in Guizhou province said in a statement that 15 people were rescued and sent to hospitals with injuries. The bus was hauled out of Hongshan Lake in a large rescue operation.

The passengers included high school students taking nationwide university entrance exams that began Tuesday.