WASHINGTON — U.S. officials say some 50 Marines are being sent to Libya to reinforce security at U.S. diplomatic facilities in the aftermath of an attack in the eastern city of Benghazi that killed the U.S. ambassador and three American members of his staff.

The Marines are members of an elite group known as a Fleet Antiterrorism Security Team, whose role is to respond on short notice to terrorism threats and to reinforce security at U.S. embassies. They operate worldwide.