Former U.S. Secretary of State Hillary Clinton opens up on what her critics have described as her achilles heel: Benghazi.br br Appearing at the National Automobile Dealers Association meeting in New Orleans, Clinton called the attack her biggest regret.br br (SOUNDBITE) (English) FORMER U.S. SECRETARY OF STATE HILLARY CLINTON SAYING:br br "You know, my biggest regret is what happened in Benghazi. It was a terrible tragedy. Losing four Americans. Two diplomats and now it's public, so I can say, two CIA operatives."br br It was in Benghazi in September 2012 when militants attacked the U.S. displomatic post and CIA compound.br br Four Americans, including Christopher Stevens, the U.S. ambassador to Libya, were killed.br br (SOUNDBITE) (English) FORMER U.S. SECRETARY OF STATE HILLARY CLINTON SAYING:br br "It was a great loss. It was a great loss to our foreign service, it was a great loss to our country.