This 2019 article by the well-known Russia scholar @kimberlymarten.bsky.social disusses the illegal activities of Wagner in Ukraine, Syria and Africa from 2014 and the fact that Prigozhin's leadership was ascertained by a Russian journalist in 2016.
https://www.tandfonline.com/doi/pdf/10.1080/1060586X.2019.1591142
https://www.tandfonline.com/doi/pdf/10.1080/1060586X.2019.1591142
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https://home.treasury.gov/news/press-releases/sm1058