It has been written about and discussed since last August that the United States government did not KNOW that Assad was behind the sarin attacks.
CIA analysts have publicly refused to trump up evidence to back Secretary Kerry’s figure pointing in front of Congress on August 30th of last year. Members of the CIA analyst crowd had learned from the Iraq invasion of 2003.
The government never even received an “intelligence assessment” from the CIA. The White House had to come up with a “government assessment” based on no evidence. Sound familiar.
The article below doesn’t point the finger directly at Turkey, but it makes the case.
“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.’”