A State Department email sent to intelligence officials and the White House situation room said the Islamist group Ansar al-Sharia claimed responsibility on Facebook and Twitter, and also called for an attack on the U.S. Embassy in Tripoli.
The document may fuel Republican efforts to show that the White House knew it was a terrorist attack, even as the U.S. ambassador to the United Nations was saying -- five days afterward -- that it appeared to be a protest gone awry.
The Obama administration's account of the Benghazi events has become a campaign issue, with Republican challenger Mitt Romney and GOP lawmakers accusing the White House of misleading Americans about the nature of the attack. But militant groups often surface after such attacks claiming responsibility and it's difficult to immediately verify such claims.
The Associated Press and other news organizations obtained the unclassified email and two related emails from government officials who requested anonymity because they were not authorized to speak about them publicly.