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.

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.

At least 56 people are dead after a bus plunged off a bridge Monday on the outskirts of Guatemala's capital, Guatemala City Fire Department spokesman Carlos Hernández told CBS News.