In the Spring of 1943 the US sent to the USSR, via the Lend-Lease Program, tens of tons of nuclear materials - including enriched uranium. [See Richard Rhodes, Dark Sun, Chapter Five, "Super Lend-Lease"].
The head of the Lend-Lease Program at that time was Harry "The Hop" Hopkins - FDR's alter ego. Hopkins is now known from the VENONA Project to have been Soviet "Agent No. 19."
N.B., The TRINITY TEST at Alamogordo, NM was in the Summer of 1945.
This info and more was what led Sen. McCarthy to say that Soviet agents infested the highest levels of government. Hollywierd commies still mock him for it.
Thanks for this info.