A bus carrying students and teachers returning from a graduation celebration crashed in Subang, Indonesia, on Saturday evening, killing at least 11 people, many of them students.

The crash occurred around 6:45 p.m. as the vehicle was returning from a hilly resort area in Bandung to Depok, West Java, the authorities said.

While the cause of the accident was still under investigation on Sunday, the West Java police said that the bus had sped out of control on a downhill road and crashed into multiple vehicles. An initial investigation revealed that the bus was apparently not fit for the road and had malfunctioning breaks.

There were 61 passengers aboard, according to the police, who said that nine people died at the scene and two died later in a hospital.