One thing about sarin is that it is relatively easy to work out its origins and the stuff in Ghouta was from Turkey who would never supply Assad.
http://www.lrb.co.uk/v36/n08/seymour-m-hersh/the-red-line-and-the-rat-line
For months there had been acute concern among senior military leaders and the intelligence community about the role in the war of Syria’s neighbours, especially Turkey. Prime Minister Recep Erdoğan was known to be supporting the al-Nusra Front, a jihadist faction among the rebel opposition, as well as other Islamist rebel groups. ‘We knew there were some in the Turkish government,’ a former senior US intelligence official, who has access to current intelligence, told me, ‘who believed they could get Assad’s nuts in a vice by dabbling with a sarin attack inside Syria – and forcing Obama to make good on his red line threat.’
The joint chiefs also knew that the Obama administration’s public claims that only the Syrian army had access to sarin were wrong. The American and British intelligence communities had been aware since the spring of 2013 that some rebel units in Syria were developing chemical weapons. On 20 June analysts for the US Defense Intelligence Agency issued a highly classified five-page ‘talking points’ briefing for the DIA’s deputy director, David Shedd, which stated that al-Nusra maintained a sarin production cell: its programme, the paper said, was ‘the most advanced sarin plot since al-Qaida’s pre-9/11 effort’. (According to a Defense Department consultant, US intelligence has long known that al-Qaida experimented with chemical weapons, and has a video of one of its gas experiments with dogs.) The DIA paper went on: ‘Previous IC [intelligence community] focus had been almost entirely on Syrian CW [chemical weapons] stockpiles; now we see ANF attempting to make its own CW … Al-Nusrah Front’s relative freedom of operation within Syria leads us to assess the group’s CW aspirations will be difficult to disrupt in the future.’ The paper drew on classified intelligence from numerous agencies: ‘Turkey and Saudi-based chemical facilitators,’ it said, ‘were attempting to obtain sarin precursors in bulk, tens of kilograms, likely for the anticipated large scale production effort in Syria.’ (Asked about the DIA paper, a spokesperson for the director of national intelligence said: ‘No such paper was ever requested or produced by intelligence community analysts.’)
Several "intelligence" outfits from various countries claimed and were reported as proven, but it is now apparent that these claims were lies fabricated to elicit the response that Obama initially came up with as a proposal.
Luckily they were not carried out.