It has been reported that there were/are 10-11 separate al Qaeda affiliated groups in Benghazi. I don't think it is surprising that whoever attacked was al Qaeda even if they consisted of more than one bunch of them. Here is how Benghazi looked just a year ago.
Benghazi: A Sea of Al-Qaeda Flags NRO Nov. 5, 2011
That still does not address the fact that anyone attacking an American Mission with the intended purpose of possibly killing dozens of Americans, (luckily that did not happen), would not want America to know who they were.
We have a President that fires off drone missiles that are too numerous to count. No one is going to run out, wave a flag, and say, shoot me next.