Three Americans who were working as contractors in Afghanistan died in a gunman's attack at the Hamid Karzai International Airport complex Thursday, the Associated Press reports.

The news agency adds:

"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."

In addition to the U.S. casualties, an Afghan citizen also died, the AP says.

Update at 3:35 p.m. ET: Three Were Defense Contractors

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