What was the CIA doing in Benghazi?

Conor Friedersdorf writes in The Atlantic that it is important to realize that the American presence in Benghazi, Libya on 9-11 was primarily a CIA mission. What were those CIA agents doing? Friedersdorf links to a CNN story that asserts

Speculation on Capitol Hill has included the possibility the U.S. agencies operating in Benghazi were secretly helping to move surface-to-air missiles out of Libya, through Turkey, and into the hands of Syrian rebels.