Gang members opened fire on a bus on the outskirts of El Salvador's capital, then doused it with gasoline and set it on fire, killing at least 14 passengers in what the president on Monday called an act of terror.
Police managed to break windows and help 13 people escape the flames but others died inside. Sixteen people suffered injuries in the late Sunday attack, officials said.
"This is an act that seeks to generate terror among the population," President Mauricio Funes said, adding that his security Cabinet was to meet to increase security in the country.
Violent youth gangs have besieged the public transit system in this small Central American nation to extort bus workers and largely impoverished commuters and travelers.
The armed assault was the deadliest attack this year on a passenger bus.
The attack took place in a gang-plagued part of the municipality of Mejicanos, just outside San Salvador, National Police Commissioner Carlos Ascencio said. At least 14 people were killed, he said.
Moments later, gang members opened fire on another bus in the same neighborhood, killing two people.
Ascencio said Monday night that eight suspects had been arrested in the bus burning, including one who was detained minutes after the attack and smelled of gasoline. Among the detained were a woman and two minors.
Earlier, Funes said seven suspects had been detained, most accused of being members of the Mara 18 street gang.
Justice Minister Manuel Melgar called the attack "a typical terrorist act," but said the motive was under investigation.
At least 217 drivers and other employees of El Salvador's public transport industry have been killed over the last year and a half in suspected gang attacks, said Catalino Miranda, president of the national federation of transport workers and businessmen. Most victims were shot to death.
He said more than 50 buses had been set on fire. Three of the burnings, including Sunday's attack, caused deaths. Two people were killed in a bus burning earlier this year, and one was killed last year.
Miranda estimated extortion demands had cost the industry some $18 million in 2009.
El Salvador has an estimated 13,500 gang members, some 5,700 of whom are in prison. It is one of Latin America's most violent countries.