Panama City, Panama ? Mechanical problems triggered a fire that raced through a bus in Panama’s capital on Monday, killing at least 18 people, injuring 25 and sending passengers jumping from the flaming vehicle, police said.

Investigators using trained dogs found no evidence of explosives on the city bus and believed some form of fuel leak was to blame for the blaze. The bus lacked an emergency exit.

“We found absolutely no trace of the presence of explosives,” said Jaime Jacome, director of Panama City’s Technical Judicial Police. “There was a lot of gasoline.”

Witnesses said they saw smoke pouring from the area around the motor seconds before the bus caught fire. Authorities were checking the charred remains of the motor and bus for clues.

Many passengers were seen scrambling to get off before flames engulfed the bus. At least 25 people managed to escape, but all were hospitalized with severe burns.