(NBC NEWS) Five U.S. soldiers killed in Afghanistan may have been the victims of a friendly-fire incident, officials said Tuesday.

The International Security Assistance Force (ISAF) said five of its servicemen had died Monday during a security operation in southern Afghanistan when their unit came in contact with enemy forces.

“Tragically, there is the possibility that fratricide may have been involved,” ISAF said in a statement. “The incident is under investigation.”

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