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.
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.
Meanwhile, the Tunisian government said it has arrested a 28-year-old Tunisian linked to the U.S. Consulate attack. Interior Ministry spokesman Tarrouch Khaled said Wednesday that the suspect, Ali Harzi, was in custody in Tunis. Khaled told The AP "his case is in the hands of justice," but did not elaborate.
Secretary of State Hillary Rodham Clinton said Wednesday that the review board she appointed to investigate the attack is "looking at everything," rather than "cherry picking one story here or one document there."
White House press secretary Jay Carney said the emails represented just one piece of information the administration was receiving at the time.
"There were emails about all sorts of information that was becoming available in the aftermath of the attack," Carney said. "The whole point of an intelligence community and what they do is to assess strands of information and make judgments about what happened and who is responsible."
Carney, traveling with President Barack Obama Wednesday on Air Force One, said the emails were unclassified and referred to assertions made on a social media site.
There were a series of three emails sent by State Department officials in Washington as events unfolded on Sept. 11. Among the recipients was the White House situation room.
The first email said that the State Department's regional security officer reported the mission in Benghazi was under attack, and that "20 armed people fired shots." It said that Ambassador Chris Stevens, who was killed in the attack, was in Benghazi, and that Stevens and four others were in the compound's safe haven.
Forty-nine minutes later, an email said that the firing at the consulate "has stopped and the compound has been cleared," while a response team was attempting to locate people.
The next message, one hour and 13 minutes after the second and some two hours after the attack began, a message reported that Ansar-al-Sharia claimed responsibility for the attack.
"Embassy Tripoli reports the group claimed responsibility on Faceboook and Twitter and has called for an attack on Embassy Tripoli," it said.
Ansar al-Sharia bragged to members of al-Qaida in the Islamic Maghreb that it was responsible for the attack, according to recordings of phone calls intercepted by U.S. intelligence. But the group has publicly denied having anything to do with the attack.
Clinton, speaking to reporters at the State Department, said, "You know, posting something on Facebook is not in and of itself evidence and I think it just underscores how fluid the reporting was at the time and continued some time to be."
She added, "What I keep in mind is that four brave Americans were killed and we will find out what happened, we will take whatever measures are necessary to fix anything that needs to be fixed and we will bring those to justice who committed these murders."
Associated Press Matthew Lee contributed to this story.
Follow Larry Margasak on Twitter at http://twitter.com/LarryMargasak
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