Gunmen boarded a bus near the Salvadoran capital and opened fire Wednesday, killing four people and wounding four others a week after a string of bus attacks brought service to a halt.

The federal prosecutor's office said the driver, the fare collector and two passengers were slain in the attack in Cuscatlan department, about 13 miles (21 kilometers) northeast of San Salvador.

Investigators said at least five presumed gang members got on the bus as it began its route toward the capital, shot the driver and then turned their guns on the passengers. The attackers escaped.

The wounded were taken to a hospital for treatment.

Another bus driver was shot and wounded in a separate attack on the northern edge of the city, prompting colleagues who work the same route to stop driving.

Bus service was all but paralyzed for several days last week after a string of attacks that killed at least eight drivers.

Drivers were warned to abandon their routes or risk being slain, an apparent tactic to pressure the government to negotiate with gangs and transfer their leaders out of maximum-security prisons.

Under heavy police and military guard, drivers returned to their routes by the weekend after authorities arrested an alleged gang leader believed to be responsible for the attacks and intimidation.

Also Wednesday, police reported that an officer was killed in an ambush while patrolling in the rural municipality of San Juan Nonualco, southeast of the capital.