Defense Secretary Leon Panetta told a Senate panel Thursday that "lack of adequate warning" prevented the U.S. military from immediately responding to the deadly attack on the U.S. Consulate in Libya last year.

Four Americans, including U.S. ambassador to Libya Chris Stevens, died in the Sept. 11 attack at the U.S. mission in Benghazi.

Panetta said there was no specific intelligence of an imminent attack on Benghazi. The consulate was among 281 other sites that had been threatened in the months leading up to the attack.