An explosion rocked a city bus just outside Guatemala's capital, killing at least four people and injuring another eight, authorities said.
"We are still investigating whether it is an attack or not, but we need the technical report to determine that," interior minister Carlos Menocal told AFP. The blast took place on Monday.
Some eyewitnesses told local media there appeared to have been an electrical short circuit. But some the police sources said it may have been an attack.
Explosives experts were investigating, officials said.
Guatemalan bus drivers paid $1.5 million in extortion money to organised crime rings in 2010, according to a police report released on Monday.
Public transport workers appear to be at particular risk. Human rights groups said that between January and November, 119 drivers and 51 transport aides were shot dead.