Washington ( CNN ) -- The four U.S. military personnel being held in Libyan government custody have been released , a U.S. Defense Department official said late Friday . The four were believed to be held by the ministry of the interior , the official , with knowledge of the situation , said on condition of anonymity . It was not immediately known why the American personnel were in Libyan custody . `` We are seeking to further ascertain the facts and ensure their release , '' the official said . The four were in Libya `` augmenting security at the U.S. Embassy in Tripoli , '' the official said . A second person -- a senior U.S. official -- also told CNN that the four military personnel had been detained . That official also spoke on condition of anonymity . The news of the detentions followed an announcement by the United States last month that it was stepping up military support for Libya 's fledgling security forces , who have been overwhelmed by militia violence and unrest since the overthrow of dictator Moammar Gadhafi . Popular sentiment against the various militias has been mounting for months in Tripoli and other parts of the country , including the eastern city of Benghazi , which has been gripped by increasing violence , including political assassinations . The chaos in Libya hit home for Americans after U.S . Ambassador Chris Stevens and three other Americans were killed on September 11 , 2012 , in an attack on the U.S. diplomatic mission in Benghazi . The United States has offered a reward of up to $ 10 million for information leading to the arrest of anyone involved in that attack . While U.S. authorities have filed charges in the case , no one has been arrested . A NATO air campaign helped overthrow Gadhafi two years ago , but ever since there has been widespread unrest and poor security . The detention of the four personnel also followed reports of a U.S. military operation in October that saw members of the elite Army Delta Force launch a raid in Tripoli to capture Abu Anas al Libi , an al Qaeda operative wanted for his alleged role in the deadly 1998 bombings of two U.S. embassies in Africa . Libya 's interim government called the capture a kidnapping and demanded an explanation from Washington about the raid . Barbara Starr reported from Washington , and Chelsea J. Carter wrote from Atlanta ; CNN 's Elise Labott contributed to this report .
Four U.S. military personnel were believed killed by the interior ministry , an official said . The soldiers were augmenting security at the U.S. Embassy in Tripoli , the official said .