"This picture is still a little fuzzy but it is starting to come into focus and it appears that there were, very early on, some indications that there was jihadist participation in the event," he said.
The Obama administration has strongly defended its public accounts of what happened in Benghazi, and said its understanding has evolved as additional information came in.
"At every step of the way, the administration has based its public statements on the best assessments that were provided by the intelligence community. As the intelligence community learned more information, they updated Congress and the American people on it," said White House spokesman Jay Carney.
Some officials said U.S. spy agencies tried to avoid drawing premature conclusions about how the violence began and who organized it.
"Unless you have very good reports that strongly suggest who was behind the attack for sure, it is prudent to be careful, because placing emphasis publicly, even tentatively, on any one group or groups too soon can lead everyone down the wrong path," said one official, who spoke on condition of anonymity.
Republicans have sought to make the shifting stories told by administration officials about the attack, and inadequate security precautions at the U.S. diplomatic site in Libya, a major issue in the presidential campaign leading up to the Nov. 6 election.
Two House Republicans said they would hold Congress' first hearing on the matter on Oct. 10.