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.
I wonder why they are still under investigation. Chelsea was banned instantly for violating the financial Fairplay. Manchester City which has more violations it is still under investigation and it is very odd.