But the U.S. intelligence community revised its assessment, saying it believes it was a planned terrorist assault.

The intelligence community now believes it was "a deliberate and organized terrorist assault carried out by extremists" affiliated with or sympathetic to al Qaeda.

Defense Secretary Leon Panetta has said the attack was "clearly" planned and conducted by terrorists, and that it "took a while" for there to be information to reach such a conclusion.

But a senior U.S. official told CNN that, within a day or so of the attack, the U.S. intelligence community had begun to gather information suggesting it was the work of extremists either affiliated with or inspired by al Qaeda groups.