If US so confident of Assad’s crimes, why don’t they publish evidence?

Is US so confident of Assad’s crimes, why don’t they publish evidence?by RussiaToday
Any possibility of British involvement in a military campaign in Syria has been effectively ruled out after British lawmakers voted down the prospect in parliament, costing the US the nation’s closest ally in a potential strike. British MP George Galloway joins RT to talk more on brewing international tensions over Syria.
RT (Russia Today) VIDEO