I Survived a Sarin Gas Attack
nytimes.com
nytimes.com
Theodore Postol of MIT analyzed the rocket trajectory and put a lot of doubt on the the location firing the rockets [0]
There is also reports that, Obama's Director of National Intelligence at the time, James Clapper came to the conclusion the gas attack did not come from the Syrian regime [1]
"Obama's Director of National Intelligence at the time, James Clapper, was able to dissuade Obama from ordering a cruise missile strike, according to a newly-published book by Mideast expert Michael Lüders. Presumably, a deciding factor was an analysis of the chemical weapons used in Ghouta, conducted by a British military lab, which found the gas to be of a different composition than the Syrian army possessed."
There are also reports of the ISIS using chemical in Syria [2]
[0] https://s3.amazonaws.com/s3.documentcloud.org/documents/1006...
[1] http://www.dw.com/en/is-assad-to-blame-for-the-chemical-weap...
[2] https://www.nytimes.com/2016/11/21/world/middleeast/isis-che...
https://www.jw.org/en/publications/videos/#mediaitems/VODMin...