A midnight bus crash killed at least 16 people on Indonesia’s main island of Java on Monday, officials confirmed. The vehicle, carrying 34 passengers, slammed into a concrete barrier on a toll road and flipped onto its side, according to a local search and rescue chief named Budiono.

The inter-province bus was traveling from Jakarta toward Yogyakarta when it overturned on a curved exit ramp at the Krapyak toll way in Semarang.

Budiono said: “The forceful impact threw several passengers and left them trapped against the bus body.”

Rescue teams arrived 40 minutes later, recovering six bodies at the scene. Another 10 victims died en route to or at the hospital. Among the 18 survivors currently being treated, five remain in critical condition and 13 are seriously injured.

Footage showed the yellow bus on its side as responders and bystanders worked amid a fleet of ambulances. Witnesses reported the bus was speeding before the driver lost control, noted Central Java Police Chief Ribut Hari Wibowo.

The driver, a substitute, survived with serious injuries and is communicating with authorities.

“We are still investigating the cause and questioning the injured substitute driver,” Wibowo said, noting that police will test him for drugs and other prohibited substances.