"There are a number of sources, but the most prestigious one is the "Dark Alliance" series of Articles in 1996. A rather respected investigative journalist name Gary Webb examined the relationship between the Contras, Central American right-wing rebel groups active from 1979 to the early 1990's, and the CIA. Congress decided to not fund the attempted overthrow of foreign governments, but the CIA pretty clearly was involved in funding such groups by letting them use government assets to smuggle drugs. This was contemporaneous with the Iran-Contra scandal, a similar attempt to fund rebellion without relying on Congressionally appropriations.
Gary Web later published a book alleging that the CIA then specifically targeted black neighborhoods in Los Angeles in the late 70's as the end point of this plot line. People were, quite justifiably outraged. There were several investigations by the local police, state officials, and Congress itself. The Justice Department report indicated that there were two major drug dealers in Los Angeles who had close ties to the CIA and were probably supplied by CIA, they were not the ones to introduce crack cocaine nor were they the biggest dealers in the neighborhood. The House Select Committee Report disagreed, suggesting that while the two drug dealers in question did have tenuous contact with CIA-affiliated individuals (most notably a smugger that the CIA bailed out once) they mostly had Non-CIA suppliers and it was unclear how much of their drugs had come from CIA-related sources.
So, it is historical fact that the CIA did get into the drug trade during the 1970's and 1980's in Central America to help fund rebel groups in nations that were identified as communist and socialist. Much of those drugs ended up in the United States, the largest drug market at the time. Some individual dealers did have contact with the CIA, but the amount of drugs coming from these questionable sources was insignificant to the overall trend. Given that the CIA had been involved with Heroine smuggling from the end of the Second World War up until the Vietnam War and involvement in the production of LSD it's unclear that the CIA had any meaningful institutional control over where these drugs ended up.
The explosion in drugs and drug-related crime coincided with the CIA's foray into cocaine smuggling, which leads some to conclude a causational link. But, frankly, it's probably the other way around. The beginning of serious drug problems left a lot of "free money" laying around to be taken advantage of by bad actors in both espionage and organized crime. And the flooding of black communities with drugs and addicts merely meant that they were existing buyers for what the Contras were selling.
Gary Webb's assertion that the CIA introduced crack to black neighborhoods and those that the FBI was involved in flooding black communities with drugs were unfounded, and at best massive exaggerations of otherwise well documented and ultimately self-defeating initiatives by US intelligence services. Though, it does provide a rather useful method to shift blame for social problems to the US Government with just enough truth to it to be plausible. Though, it's probably untrue given the other embarrassing interventions in the drug trade we know that the CIA was involved in."
For further reading:
There is
Cocaine Politics : Drugs, Armies, and the CIA in Central America by one Peter Scott,
The Politics of Heroin in Southeast Asia by Alfred McCoy, and the infamous (but not peer reviewed and not completely accurate)
Dark Allianceby Gary Webb.
For a more general understanding of what the CIA has been up to you might want to read
The Agency: The Rise and Decline of the CIA by John Ranelagh.
Check out what his editor Jerry Ceppos, who initially supported all of Webb's claims, wrote himself after an internal investigation of the series.
https://web.archive.org/web/19971119070955/http://www.sjmercury.com/drugs/column051197.htm