The U.S. government has begun its investigation into how a well-armed group of men was able to kill the American ambassador to Libya in an assault on the US consulate in Benghazi on Tuesday. The attack came amid a protest against a YouTube movie made in America and aimed at insulting the prophet Muhammed. It was the first time a US ambassador was killed in the line of duty in 33 years. Ambassador Christopher Stevens had been the chief diplomatic liaison with Libyan rebels struggling to oust Muammar Gaddafi in 2011 and, prior to that, had served as the second-ranking diplomat in Libya during Gaddafi's reign. Patty Culhane reports from Washington DC.