The assessment follows a scathing report by an independent State Department accountability review board that resulted in a top security official and three others at the department stepping down.
The attack, in which U.S. Ambassador Christopher Stevens died, has put diplomatic security practices at posts in insecure areas under scrutiny and raised questions about whether intelligence on terrorism in the region was adequate.
The Senate report said the lack of specific intelligence of an imminent threat in Benghazi "may reflect a failure" in the intelligence community's focus on terrorist groups that have weak or no operational ties to al Qaeda and its affiliates.
"With Osama bin Laden dead and core al Qaeda weakened, a new collection of violent Islamist extremist organizations and cells have emerged in the last two to three years," the report said. That trend has been seen in the "Arab Spring" countries undergoing political transition or military conflict, it said.