On Sept. 11, 2012, an attack at the U.S. diplomatic compound in Libya left four people dead, including U.S. Ambassador Christopher Stevens.
"It would certainly be the attack on our facility in Benghazi, and the loss of two State Department personnel and two CIA contractors from the terrorist attack and the terrible consequences of that," Clinton said when asked what her biggest regret was.
"It's very, very painful and it was certainly the biggest regret that I had as secretary of state," she added.
"They weren't the only people that we lost, but we lost them in such a terrible, senseless, terrorist action that, you know, it's just deeply sorrowful and it went on for hours, because the CIA annex was attacked after the State Department facility was attacked," she said of the assault.