* White House says claim should not be taken as fact
* State Dept. ops center sent emails to top officials
* Messages do not resolve whether attack was planned
* Spy agencies confirmed likely militant role Sept. 28 (Recasts with Clinton and White House comments)
By Mark Hosenball
WASHINGTON, Oct 24 (Reuters) - U.S. Secretary of State Hillary Clinton said on Wednesday a Facebook post in which an Islamic militant group claimed credit for a recent attack on the U.S. mission in Benghazi, Libya did not constitute hard evidence of who was responsible.
"Posting something on Facebook is not in and of itself evidence. I think it just underscores how fluid the reporting was at the time and continued for some time to be," Clinton said during an appearance with the Brazilian foreign minister at the State Department.
Reuters reported on Tuesday that an official email showed officials at the White House and State Department were advised two hours after attackers assaulted the U.S. diplomatic mission on Sept. 11 that an Islamic militant group had claimed credit.
The email, obtained by Reuters from government sources not connected with U.S. spy agencies or the State Department and who requested anonymity, specifically mentioned that the Libyan group called Ansar al-Sharia had asserted responsibility for the attacks.
That and two other emails also made available showed how U.S. diplomats described the attack, even as it was still under way, to Washington.
Clinton, responding to a reporter's question about the emails, noted that a State Department investigation was under way.
"The independent accountability review board is already hard at work looking at everything, not cherry picking one story here or one document there, but looking at everything - which I highly recommend as the appropriate approach for something as complex as an attack like this," she said.
"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."
White House spokesman Jay Carney, asked about the emails, noted that Ansar al-Sharia had later denied responsibility for the attack.
"This was an open-source, unclassified email about a posting on a Facebook site. I would also note I think that within a few hours, that organization itself claimed that it had not been responsible. Neither should be taken as fact -- that's why there's an investigation under way," he told reporters traveling with President Barack Obama aboard Air Force One to Iowa.
U.S. Ambassador Christopher Stevens and three other Americans were killed in the Benghazi assault, which Obama and other U.S. officials ultimately acknowledged was a "terrorist" attack carried out by militants with suspected links to al Qaeda affiliates or sympathizers.
Administration spokesmen, including Carney, citing an unclassified assessment prepared by the CIA, maintained for days that the attacks likely were a spontaneous protest against an anti-Muslim film.
While officials did mention the possible involvement of "extremists," they did not lay blame on any specific militant groups or possible links to al Qaeda or its affiliates until intelligence officials publicly alleged that on Sept. 28.
There were indications that extremists with possible al Qaeda connections were involved, but also evidence that the attacks could have erupted spontaneously, they said, adding that government experts were cautious about pointing fingers prematurely.
U.S. intelligence officials have emphasized since shortly after the attack that early intelligence reporting about the attack was mixed.
MISSIVES FROM LIBYA
The records obtained by Reuters consist of three emails dispatched by the State Department's Operations Center to multiple government offices, including addresses at the White House, Pentagon, intelligence community and FBI, on the afternoon of Sept. 11.
The first email, timed at 4:05 p.m. Washington time - or 10:05 p.m. Benghazi time, 20-30 minutes after the attack on the U.S. diplomatic mission allegedly began - carried the subject line "U.S. Diplomatic Mission in Benghazi Under Attack" and the notation "SBU", meaning "Sensitive But Unclassified."
The text said the State Department's regional security office had reported that the diplomatic mission in Benghazi was under attack. "Embassy in Tripoli reports approximately 20 armed people fired shots; explosions have been heard as well," it said.
The message continued: "Ambassador Stevens, who is currently in Benghazi, and four ... personnel are in the compound safe haven. The 17th of February militia is providing security support."
A second email, headed "Update 1: U.S. Diplomatic Mission in Benghazi" and timed 4:54 p.m. Washington time, said that the Embassy in Tripoli had reported that "the firing at the U.S. Diplomatic Mission in Benghazi had stopped and the compound had been cleared." It said a "response team" was at the site attempting to locate missing personnel.
A third email, also marked SBU and sent at 6:07 p.m. Washington time, carried the subject line: "Update 2: Ansar al-Sharia Claims Responsibility for Benghazi Attack."
The message reported: "Embassy Tripoli reports the group claimed responsibility on Facebook and Twitter and has called for an attack on Embassy Tripoli."
While some information identifying recipients of this message was redacted from copies of the messages obtained by Reuters, a government source said that one of the addresses to which the message was sent was the White House Situation Room, the president's secure command post.
Other addressees included intelligence and military units as well as one used by the FBI command center, the source said.
It was not known what other messages were received by agencies in Washington from Libya that day about who might have been behind the attacks.
Intelligence experts caution that initial reports from the scene of any attack or disaster are often inaccurate.
By the morning of Sept. 12, the day after the Benghazi attack, Reuters reported that there were indications that members of both Ansar al-Sharia, a militia based in the Benghazi area, and al Qaeda in the Islamic Maghreb, t h e North African affiliate of al Qaeda's faltering central command, may have been involved in organizing the attacks.
One U.S. intelligence official said that during the first classified briefing about Benghazi given to members of Congress, officials "carefully laid out the full range of sparsely available information, relying on the best analysis available at the time."
The official added, however, that the initial analysis of the attack that was presented to legislators was mixed.
"Briefers said extremists were involved in attacks that appeared spontaneous, there may have been a variety of motivating factors, and possible links to groups such as (al Qaeda in the Islamic Maghreb and Ansar al-Sharia) were being looked at closely," the official said. (Additional reporting by Susan Cornwell and Andrew Quinn; Editing by Mary Milliken and David Storey)