U.S. defense officials say that special operations forces in Afghanistan last month attempted to rescue two professors kidnapped from the American University of Afghanistan in Kabul.
Pentagon press secretary Peter Cook said the rescue mission was approved by Defense Secretary Ash Carter and authorized by the president.
Officials say the mission was delayed one day because of questions about the intelligence and whether the hostages, an American and an Australian, were there.
"Unfortunately, the hostages were not at the location we suspected," Cook said, adding that the operation was unsuccessful. "During the mission, U.S. forces engaged and killed a number of hostile forces. No U.S. personnel or civilians were harmed."
Cook refused to provide any more details about the operation "in order to protect the safety of hostages and operational security."
