Apparently a former mafia member confessed to being part of the plot to murder John Paul I. God knows if it is true, but it is interesting corroboration.
https://novusordowatch.org/2019/10/ex-mobster-murder-john-paul1/
Unfortunately, testimony from a career criminal (trying to sell a book, BTW) has very little credibility. He has no compunction about lying and such a thing would help him sell more books. Maybe it's true; maybe it's not.