At least 51 people are dead after their bus plunged off a bridge Monday on the outskirts of Guatemala’s capital.

The bus collided with other vehicles on a main bridge in Guatemala’s capital before falling into a ravine early Monday morning.

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, which happened in the early hours of Monday, local time.