You just called General Mattis a liar, as well as the UN investigators of Ghouta, as well as....
Well .. it's of course possible that he's lying. It's also possible that he's been given fake intel.
For those who may not have been following this: here's what General Mattis said (from the CSPAN transctipt - ALL CAPS unfortunately).
"I AM CONFIDENT THE SYRIAN REGIME CONDUCTED -- A CHEMICAL ATTACK ON INNOCENT PEOPLE IN THIS LAST WEEK, YES, ABSOLUTELY CONFIDENT OF IT. WE HAVE THE INTELLIGENCE LEVEL OF CONFIDENCE WE NEEDED TO CONDUCT THE ATTACK."
All I can say is: if there is evidence so damning that it can change the tenor of Mattis's public statements in a single day, let it be released.