I'm sure they have their "proof". Or else they wouldn't make such insane claims.
Do I believe it? Fuck yes I believe it. There is so much "secretive" shit going on with all big countries we wouldn't be able to count them all up. I'm 100% confident that the US made this program to spy, i'm 100% confident the US had part in the assassinations of some of the Nuclear scientists in Iran.
The US, and many other countries do NOT want Iran having these weapons so in the end the federal staff we see all the time "president, etc" will talk and do the sanctions.
While the other staff will set up missions, viruses, etc, to slow down or halt the progress of Irans program without any "connections" back to the US.
This isn't conspiracy, this is just what governments with unlimited money does.