KABUL (AP) - An unknown gunman shot dead six local workers from the main American military base north of Afghanistan’s capital and wounded three others, an Afghan official said Friday.

The nine workers, all Afghan nationals, were on their way home late Thursday when a gunman riding a motorcycle opened fire on them about 500 meters (about a quarter mile) from Bagram air base, Parwan provincial governor’s spokeswoman Wahida Shahkar said.

Shahkar said the gunman escaped.

No insurgent groups immediately claimed responsibility for the attack but Taliban and Islamic State group fighters are active in northern areas of Afghanistan.

IS claimed responsibility for firing five rockets from a vehicle at Bagram air base on April 9 but there were no casualties.

Shahkar said Afghan national security forces began an investigation to find the person who attacked the workers, who provide cleaning services at the base.

The Afghan government and Taliban are in the process of exchanging prisoners as part of a peace deal signed by the U.S. and the Taliban on Feb. 29 in Doha.

The release of up to 5,000 Taliban prisoners and 1,000 government personnel ahead of intra-Afghan negotiations is a condition of the U.S.-Taliban deal.