All remaining U.S. government employees were evacuated from Benghazi, Panetta said.

Beefing up security

There will "always be tension" between how much security is adequate and how much would create a "bunker-like mentality" at global posts, Panetta said. The answer is not to assign the military to run a "fire house" next to every U.S. diplomatic location.

Panetta noted that Congress, too, plays a role in the security of the nation's diplomatic missions, and that the Department of Defense faces the prospect of sequestration -- which would result in billions of dollars of cutbacks to the Defense budget -- on March 1. "If Congress fails to act, sequestration is triggered," he said.

Panetta also said there must be some reliance on host countries to help with security. The attack in Benghazi "raises concerns" about whether that's always possible. Libya's government has been on shaky ground since Libyan leader Moammar Gadhafi was ousted in 2011.

The defense secretary's testimony comes a few weeks after Clinton told lawmakers that the State Department was moving fast to beef up security at U.S. posts worldwide.

During her testimony, Clinton teared up as she recounted meeting the arrival of the caskets containing the bodies of Stevens and former Navy SEAL commandos Tyrone Woods and Glen Doherty, both of whom had been working as diplomatic security officers. Sean Smith, the fourth American killed, was an information management officer.

Panetta said an FBI team that includes CIA and DOD members "has made very good progress" in identifying the attackers.