An intriguing CIA history article, which is too long to post in sections.
https://www.cia.gov/library/center-for-the-study-of-intelligence/csi-publications/csi-studies/studies/vol47no1/article02.html
Comparing this with, say, Iraq, it appears that the CIA relies upon much more thorough research and analysis after long ago events than when advising the US President before immediate ones. 
Original documents on Joseph Davies’ mission to Moscow for Roosevelt “behind Churchill’s back” are here. http://yalepress.yale.edu/yupbooks/excerpts/butler_stalin.pdf