The U.S. is sending spies and drones to Libya to speed the search for suspected al-Qaida sympathizers who killed U.S. ambassador Chris Stevens and three other Americans.
The investigation is complicated by a chaotic security picture and limited American and Libyan intelligence resources.
The U.S. has already sent an FBI investigation team to Libya and increased drone flights over the country, hunting those who turned a demonstration over an anti-Islamic video into a violent attack on the U.S. consulate in Benghazi.
But U.S. officials say the Americans have found no evidence pointing conclusively to a particular group or to indicate Tuesday's attack was planned. They spoke anonymously because they were not authorized to discuss the investigation publicly.