Hours before dawn, Julio Arrivillaga and Catalina Pérez Molina boarded a bus with other residents in the center of the humble village of Santo Domingo Los Ocotes in Guatemala for what should have been an hour-long ride to Guatemala’s capital.

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 51 people were killed as a bus plunged into a ravine early Monday in Guatemala City, officials said. At least 20 more were seriously injured, they said. City officials said the incident occurred when the bus veered off a highway and went into a ravine.