Gunmen boarded a bus in a town near the Salvadorean capital and opened fired on passengers on Friday, killing six, including three law enforcement officials, police said.
The Salvadorean Attorney General’s Office said a police investigator and two prison guards who worked at a maximum security prison were among the victims.
Another five people were wounded during the attack in the town of San Luis Talpa near San Salvador, Salvadorean Police Director Rigoberto Pleites said.
Pleites said witnesses told police the assailants were gang members dressed in uniforms similar to those issued to state road maintenance workers.
He said investigators have not confirmed the attackers belonged to a gang.
Authorities did not provide a motive for the attack.
However, Police Assistant Director Mauricio Ramirez said there have been threats of increased violence in the Central American country in the coming days.
In 2012, the Mara Salvatrucha and the rival Mara 18 street gangs agreed to a truce.
Homicides dropped from 12 per day in 2011 to an average of 6.8 per day last year.
However, in the first three months of this year, the homicide rate has spiked back up to about 8.9 killings per day.
Ramirez said officials have ordered that no police officers take time off in the coming days after at least 21 people were killed in the country in less than 24 hours.
Also on Friday, authorities found the bodies of six boat maintenance workers who had gone the previous night in the Pacific coast city of Puerto Barillas.
The men’s bodies were found half-buried in a clandestine grave in the town of Jaquilisco and members of the MS-13 gang are suspected in the killings, police said.