There was no "real-time information" to act on, Defense Secretary Leon Panetta said Thursday in a news briefing.
A defense official noted while a drone was overhead in the area, the images looking down into the compound revealed fires and chaos, but not enough information to determine what was going on, CNN reported.
"We didn't have good eyes on the situation," the official said.
He added such situations normally deteriorate over days, not hours, which was not enough time to send in even quick-reaction teams.
Panetta termed criticism about the delayed reaction to the Benghazi consulate attack "Monday-morning quarterbacking."
A closed-door hearing on security at the mission and threat reports in Libya will be held next month by the Senate Intelligence Committee, the committee announced Thursday.
Investigations by several government agencies are continuing.