Three American contractors and one Afghan man were killed Thursday in what one Afghan official described as an "insider attack," according to Reuters.
"It is unclear yet why he shot these advisors and no-one else was there to tell us the reason," the official said on condition of anonymity because he was not authorized to speak to the media.
U.S. defense officials told the Associated Press that the shooting, which also killed a local Afghan citizen, occurred in the early evening hours at the North Kabul International Airport complex.
The officials did not release any further details, including the identities of the contractors, citing the early stages of the investigation.
The status or identity of the gunman was not immediately known, although the air force official told Reuters he was an Afghan soldier.
The Taliban later claimed responsibility for the attack which killed three U.S. contractors.
Breaking: Taliban claimed responsibility for the insider attack at the Kabul militry airport in which 3 US contractor were killed last night
Taliban spokesman Zabihullah Mujahid said in a statement the attacker, Hessanullha from Laghman province, wore a police uniform and had infiltrated the ranks of Afghan forces to stage the attack, the Associated Press reported.