Guatemala is calling for outside help to investigate the attack
Fifteen people whose bodies were found in the charred shell of a bus in eastern Guatemala on Sunday had been shot first, investigators have said.
Police believe the fire was set deliberately and that the deaths were drug-related, a spokesman said.
Apart from a Dutch national, all on board were Nicaraguan, officials say.
Nicaraguan authorities have called on their Guatemalan counterparts to conduct a "transparent and efficient" investigation into the deaths.
A police spokesman in Guatemala City said parts of the bus, which set out from the Nicaraguan capital, Managua, on Friday, had been torn away as if someone were looking for secret compartments, the Associated Press reported.
Officers are also trying to find out what happened to a 16th person on board when the bus journey began but whose body was not found at the scene.
Guatemala's Attorney General Amilcar Velezquez said he would be asking for international help to identify the victims, and who was behind the incident.
The bus was found in the Zacapa area, some 140km (87 miles) from the capital, where drug gangs are known to operate.
Police said the people on board the bus were found still in their seats, their bodies so badly burned that even their gender could not be initially established.
Police officers said traces of petrol had been found on and around the bus, which formed part of a regular services between the two capitals.
"We are calling on the Guatemalan authorities to be transparent and efficient so there is no shadow of a doubt in the case, as happened with the deputies from El Salvador ," Nicaragua's Attorney General Julio Centeno said.
This was a reference to the murder in 2007 of three Salvadorean politicians and their driver near Guatemala City.
Four Guatemalan policemen arrested in connection with the killings were themselves murdered in jail.