Ansar al Sharia behind Benghazi?

Ex-CIA chief David Petraeus is set to tell House and Senate committees the U.S. suspected Ansar al Sharia was responsible for the Benghazi attack. What is it, and why wasn't it identified as a prime suspect two months ago?

Source: http://edition.cnn.com/2012/11/16/politics/benghazi-ansar-al-sharia/index.html?eref=edition

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