Seymour Hersh: Whether you like it or not, the only game in town is Bashar Assad.

Full transcript of interview here.

The article written  by Hersh at London Review of Books on the Ghouta sarin attack is here.
IMO, the following from the article says it all:

One high-level intelligence officer, in an email to a colleague, called the administration’s assurances of Assad’s responsibility a ‘ruse’. The attack ‘was not the result of the current regime’, he wrote. A former senior intelligence official told me that the Obama administration had altered the available information – in terms of its timing and sequence – to enable the president and his advisers to make intelligence retrieved days after the attack look as if it had been picked up and analysed in real time, as the attack was happening. The distortion, he said, reminded him of the 1964 Gulf of Tonkin incident, when the Johnson administration reversed the sequence of National Security Agency intercepts to justify one of the early bombings of North Vietnam.