There's a clause, it basicly makes it very hard to sue "City Hall" It's called sovereign immunity. I doubt he can be sued personally (you'd have to prove forethought and intentional malice), rather the municipality would be liable then you run into sovereign immunity.
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You do not understand defamation law. Don't think this guy has vroseed the line, yet; but he was getting close to it.
It is a lot easier to sue city hall than you think. Cities have no immunity for defamation or civil rights claims.
If it turns out to be true that both of these women have lied about this "alleged" rape, I pray that both are arrested. I also hope that the lax players sue them for restitution, if that's possible, for possibly branding them as rapists forever. Stripper Crystal knows all about restitution and if she has to pay for the rest of her life, so be it. What the media, Kimberly G., the PA, and everyone else involved have done is dispicable. I can't stand the talking heads, and NG and KG are the worst.
The state of NC tried the "sovereign immunity" defense a few years ago with regard to an "intangibles tax" they were levying and with differences in how they levy income tax on retirees from state employment (state retirement plan - no tax, "optional retirement plan" TIAA/CREF - pay tax). They lost. If it didn't work for the state it won't work for the city.