A firebomb placed in the rear of a crowded bus in the Colombian port city of Barranquilla exploded, killing at least 15 passengers and wounding 10 others. Two other buses also were set afire, but police said there were no casualties. Officials said no individual or group claimed responsibility, but they suspected left-wing guerrillas. Police said they were hunting for a young man and woman who got off the bus shortly before the blast. Several other buses have been targets of attacks that may have been prompted by a recent government-ordered 22% boost in bus fares.