U.S. special operations forces mounted an unsuccessful attempt to rescue two kidnapped American University of Afghanistan professors last month, the Pentagon confirmed Thursday afternoon.

“In August, at the recommendation of Secretary Carter, President Obama authorized U.S. forces to conduct a mission in Afghanistan, aimed at recovering two civilian hostages,” the Pentagon spokesman Peter Cook said in a statement. “Unfortunately, the hostages were not at the location we suspected. During the mission, U.S. forces engaged and killed a number of hostile forces.”

The statement added: “No U.S. personnel or civilians were harmed. We will not provide further information on this mission in order to protect the safety of hostages and operational security. Military hostage rescue operations are inherently sensitive and dangerous and careful deliberation went into this mission. The United States military remains fully prepared to take extraordinary steps to protect American citizens anywhere in the world.”

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