The US is sending more spies, Marines and drones to Libya, trying to speed the search for those who killed the US ambassador and three other Americans, but the investigation is complicated by a chaotic security picture in the post-revolutionary country, and limited American and Libyan intelligence resources.
The CIA has fewer people available to send, stretched thin from tracking conflicts across the Middle East, Africa and Asia.
And the Libyans have barely re-established full control of their country, much less rebuilt their intelligence service, less than a year after the overthrow of dictator Moammar Gaddafi.
The US has already deployed an FBI investigation team, trying to track al-Qaida sympathizers thought to be responsible for turning a demonstration over an anti-Islamic video into a violent, coordinated militant attack on the US consulate in Benghazi.
Ambassador Chris Stevens, and three other embassy employees were killed after a barrage of small arms, rocket-propelled grenades and mortars tore into the consulate buildings in Benghazi on Tuesday, the 11th anniversary of 9/11, setting the buildings on fire.
President Barack Obama said in a Rose Garden statement the morning after the attack that those responsible would be brought to justice. That may not be swift. Building a clearer picture of what happened will take more time, and possibly more people, US officials said on Friday.
Intelligence officials are reviewing telephone intercepts, computer traffic and other clues gathered in the days before the attacks, and Libyan law enforcement has made some arrests. But investigators have found no evidence pointing conclusively to a particular group or to indicate the attack was planned, White House spokesman Jay Carney said, adding, "This is obviously under investigation."