The Pentagon dispatched a contingent of Marines to Libya, moved warships toward its coast, and planned to use drones in a stepped up search for those responsible for an attack on a U.S. consulate that killed the American ambassador and three others. A senior military official told CNN that the Pentagon and other agencies would review a video of Tuesday's assault by heavily armed militants on the diplomatic compound in Benghazi that killed Ambassador Chris Stevens, another diplomat and two security personnel. The official had not viewed the video and provided no details about its source. American drones also were expected to join the hunt for potential targets. They would be part of "a stepped-up, more focused search" for a particular insurgent cell that may have been behind the attack, the official said. The unmanned surveillance aircraft are expected to fly over Benghazi and other areas of eastern Libya to look for militant encampments, another official said, adding that drones would gather intelligence and hand the information to the Libyans to strike any targets. A senior Libyan official told CNN in June that U.S. controllers were already flying the unmanned craft over suspected militant training camps in eastern Libya because of concerns about rising activity by al Qaeda and like-minded groups. About 50 Marines from a rapid reaction force headed to Tripoli to enhance security. The unit is trained to retake or guard diplomatic installations and other facilities in troubled regions. And a pair of Navy destroyers moved toward the Libyan coast, two U.S. officials told CNN late on Wednesday. The USS Laboon and USS McFaul are each equipped with tomahawk cruise missiles that could be fired if a strike were ordered. "These ships will give the administration flexibility" if the Obama administration orders action against targets in Libya, a senior official said. The Navy typically maintains up to four Aegis-equipped missile ships in the eastern Mediterranean and the McFaul and Laboon were part of that deployment. Separately, the FBI opened an investigation, saying that it would coordinate with other federal agencies and Libyan authorities. The FBI would not speculate on facts or circumstances of the attack.