The German Wikipedia article "conspiracy theory" ("Verschwörungstheorie") says, roughly translated by me:
Since about the early 1960s, the US government does not support conspiracy theories any more. On the contrary, it uses the concept to discredit unwelcome heterodox knowledge. It used this instrument for the first time in 1967, when the CIA tried to depict as untrustworthy the spreading whispered criticism of the report of the Warren Commission, which had named Lee Harvey Oswald as the sole murderer of Kennedy.
The reference provided for this is the book Conspiracy Theory in America by Lance deHaven-Smith (2014).
The English Wikipedia article "conspiracy theory" does not make the same claim, but it does say:
Originally a neutral term, since the mid-1960s in the aftermath of the assassination of US President John F. Kennedy, it has acquired a derogatory meaning, implying a paranoid tendency to see the influence of some malign covert agency in events.
referencing the book 20th Century Words by John Ayto (1999).
I'm aware that the quoted text from the German Wikipedia is weasely and jargony, and as a whole the text is unsupportable. But it does contain one clear claim, namely that the derogatory meaning of the term "conspiracy theory" noted by the English Wikipedia did not just come about by chance, but was specifically promoted by the CIA.
I would like to know whether this claim, that the CIA facilitated (by means unknown to me) a semantic change of the term "conspiracy theory" for the worse, is supported by facts.
The supposed purpose of this would have been to discredit critics of the report of the Warren commission, but this is not the main thing I'm interested in.