A large truck slammed into a crowd gathered on a dark highway in western Guatemala, killing 18 people and leaving bodies scattered on the roadway, firefighters said.
Chacaj said that confusion during the early response to the accident had led his department to erroneously count the casualties.
Eighteen people died in the crash, with another 15 people being treated for injuries at local hospitals.
Bodies of the victims of the accident in Nahualá.
The semi-trailer truck fled the scene after the accident.
The truck was apparently travelling without its headlights on.
The number of fatalities has been reported at 18, with at least another nine people admitted to hospitals in a critical condition, Public Health Minister Carlos Soto said on local radio.More news: Quick Read: Cheollima Civil Defense: Group claims raid on North Korea's embassy
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The crash killed 18 people, including an eight-year-old girl, the Attorney General's office said. Photos showed bloodstains, shoes and other debris on the asphalt amid yellow police tape.
Morales said authorities were working to support the victims' families as he offered "heartfelt condolences".
"We sympathize with the injured, their families, and the families of the deceased, hoping they will recover from this tragedy", a statement released on Twitter said.
"We are now coordinating actions to provide all the support to the victims' families", he said.
Castillo said investigators were initially prevented from getting to the scene by angry residents and relatives of the victims, and were given access only after several hours of talks with community leaders.
Traffic rules are frequently ignored in Guatema la, a Central American country of some 16.5 million people.