In addition to Mr Stevens, information technology specialist Sean Smith and security workers and ex-Navy Seals Tyrone Woods and Glen Doherty were killed.
The White House initially said the attack stemmed from anti-American protests over a crude video produced in the US that was deemed insulting to Islam.
Analysis Rana Jawad, BBC News, Libya
This is the second raid of its kind to be carried out by American special forces on Libyan territory.
Ahmed Abu Khattala is an Islamist militia commander who heads the Benghazi-based Ansar Al Sharia group. A source close to the hard-line Islamist group told the BBC they knew he was captured two days ago but were not aware the Americans were going to take him.