At least 54 people were killed after a bus veered off a bridge into a ravine in Guatemala City on Monday, according to the country’s National Institute of Forensic Sciences.

The fire department said the driver lost control of the bus and collided with several small vehicles before plunging over the precipice.

Police in Guatemala are investigating what could have caused a passenger bus to crash through a guardrail and tumble into a ravine. At least 55 of the around 70 people on board were killed in the accident,