I have worked on NSC statements in this regard. They are based on the declassification of intelligence and authorized release of sensitive information. US officials and our allies undoubtedly integrated available evidence and intelligence into these public statements.
Yet Trump is dangerously echoing Russian propaganda instead of standing with his own team. His statements that he hasn’t seen proof do not track with what his own officials are saying, not to mention the statements by US allies.
More broadly, Trump’s refusal to condemn a Russian chemical weapon attack on an opposition figure signals that Russia can get a free pass on just about anything — and Putin won’t hear a peep from POTUS. That puts every one of the Kremlin’s perceived enemies at risk — Putin feels emboldened to tick through his macabre to-do list of threats to eliminate.