Like I said, read the McCoy book. A brief introduction here<
http://en.wikipedia.org/wiki/Alfred_W._McCoy>
As far as crack in LA, read Gary Webb's Dark Alliance series of articles published in the San Jose Mercury News in '96. <
http://www.narconews.com/darkalliance/drugs/start.htm> The series was a well crafted piece of work, but also a grand example of journalistic caving on the part of the Mercury News. Funny how Webb was found dead with not one, but two taps to the head, yet his death was ruled a suicide. A brief bio of Webb.<
http://en.wikipedia.org/wiki/Gary_Webb>
Another interesting history along these lines is Martin Lee's book Acid Dreams, detailing the CIA's involvement with psychedelic and other drugs. <
http://www.amazon.com/Acid-Dreams-Complete-History-Sixties/dp/0802130623> Martin Lee's bio <
http://en.wikipedia.org/wiki/Martin_A._Lee>
These are all respected, conscientious academics and journalists. Their work is corroborated by other people and papers, including information released by various government agencies over the years(A good starting place is to look at the findings of the Church Committee back in 1976).
As far as the question "who, exactly", well, in many cases that's the kicker. While various lower level folks have taken the fall for various misdeeds (Oliver North being the most well known), the fact of the matter is that the question of who is not known and may not be known for a long time, if ever. In many cases the best you can do is assign the blame to an agency like the CIA, which gets slapped on the wrist, reorganizes operations and moves on.
Can we say the president knows this, perhaps in some cases throughout the years. Bush the Elder's long involvement with the CIA leads me to believe he was a fairly hands on type of person when it came to these matters, especially when he was serving under Reagan. Many credible speculations have posited that Iran Contra led back to Bush Sr. and that is why North took the fall (and came out surprisingly well, a darling of the right and beneficiary of the right's media machine). But in the end, the question you ask is one that echoes down the pages of history.