The Americans have nothing to gain and everything to lose by presenting false or manicured evidence.
Additionally everytime they reveal more evidence they uncover their sources which are now compromised.
The Intelligence staff of the USA will doubtless talk to the Intelligence staff of other countries and substantiate their information, these other countries Int. staff can then reassure their UN representatives. One thing you can be sure of is that the USA won't give away to Jo Public any more than they have to, for obvious reasons.
Personally I thought the new evidence was both credible and compelling.