An Afghan soldier has reportedly killed three U.S. contractors and wounded a fourth at Kabul's military airport.
An Afghan air force official said on January 29 it is unclear what the soldier's motive was for the killings.
No information on the fate of the shooter was immediatly known.
The U.S.-led international force in Afghanistan confirmed the shooting, as did a U.S. defense official.
The Afghan air force official said an investigation is under way.
Several so-called "insider attacks" against international soldiers stationed in Afghanistan have occurred in recent years, culminating in the August killing of U.S. Major General Harold Greene.
The attacks led foreign forces to reduce their interaction with Afghan security forces.
The international troops' Resolute Support mission, which began on January 1,involves some 12,000 soldiers and focuses on training Afghan army troops.