Not officially confirmed yet, however - 

The US ambassador to Libya and three other embassy staff have been reported killed by an armed Islamist mob that stormed and set fire to the United States consulate in the Libyan city of Benghazi.

The death of the US ambassador, Christopher Stevens, was reported by Reuters, who had spoken to a Libyan official. It had not been confirmed.

It was not clear if the ambassador was in his car or the Libyan consulate when the attack occurred.