Washington described a Sept. 11 attack on the U.S. consulate in Benghazi, Libya, as an act of terrorism. The attack left U.S. Ambassador to Libya Christopher Stevens and three of his staff members dead.
Pentagon spokesman George Little said the FBI and Department of Defense were coordinating with the investigation in Benghazi.
"The intelligence community, the State Department, FBI -- the full range of capabilities of this government -- have been used to try to determine what happened in this tragic incident," he said.
Some U.S. lawmakers have accused the State Department of overlooking security concerns in Libya before the attack.
ABC News reports Friday it obtained internal State Department messages from May that suggest some security requests by the Embassy of Libya were turned down.
U.S. Reps. Jason Chaffetz, R-Colo., and Darrell Issa, R-Calif., sent a letter to U.S. Secretary of State Hillary Clinton this week calling on her to examine reports of security concerns ahead of the attacks.
The House Oversight and Government Reform Committee is to have hearings on the Benghazi attack next week.