The U.S. is sending more spies, Marines and drones to Libya, trying to speed the search for those who killed the U.S. 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. Much of the team dispatched to Libya during the revolution had been sent onward to the Syrian border, U.S. officials say.

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 Gadhafi.