WASHINGTON (AP) — U.S. defense officials say three American contractors were shot to death by a gunman in the Afghan capital of Kabul.

One official said the shooting happened Thursday at the North Kabul International Airport complex in the early evening hours. Officials said a local Afghan national was also killed.

It was not immediately clear who did the shooting or whether the shooter was a member of the Afghan security forces.

The U.S. defense officials spoke on condition of anonymity because the incident was in the early stages of investigation.