There is another possibility: We shipped them from Benghazi through Turkey to set up a false flag operations carried out by the rebels in order to blame Assad. Assad has long known that the EU and the US has just been looking for the excuse of WMD use in order to intervene, so why would he give them the rationale on a sliver platter?
If they came from Libya, then Qaddafi got them from Saddam! I think it is common knowledge that Syria did have these kinds of weapons long before the Benghazi thing happened. I see no "win-win" for Obama for conniving such an operation. He sure backtracked on the "red-line" with Syria which, I think, gave Assad the idea that Obama was too much of a coward to do what he threatened to do at the start. He figured the LAST thing Obama would want is to get involved in ANOTHER mid-east war now that we were technically out of Iraq and soon from Afghanistan.