The doctor, Ziad Abu Zeid, said Stevens was brought to the Benghazi Medical Center by Libyans on Tuesday night with no other Americans and that initially no one realized he was the ambassador. He tried for 90 minutes to revive him.
Stevens was a career diplomat who spoke Arabic and French and had already served two tours in Libya, including running the office in Benghazi during the revolt against Gaddafi.
Five other US ambassadors have been killed in the line of duty, the last being Adolph Dubs in Afghanistan in 1979.
President Barack Obama today condemned the attack and said he has ordered increased security at diplomatic posts around the world.
'It's especially tragic that Chris Stevens died in Benghazi as it's a city that he helped to save,' Obama said outside the White House.