Do you refer to the financial fairplay?
I am not aware of any illegal activities done from the City that should be under investigation. The only one is the financial fairplay which I think that City doesn't follow the rules.
IF big IF I can recall correctly.
Manchester City is not only under investigation for the financial Fairplay.
I said only because the other accusations IF I can remember correctly is for fraud, they paid their coach with money outside the society.
More info here,
https://edition.cnn.com/2019/03/08/football/manchester-city-uefa-investigation-spt-intl/index.htmlThe case of the city is huge is about violations for 5 years.