They might suspect you had a hand in the reporting, but I wouldn't have thought they could start a lawsuit unless they had some evidence you had.
If you simply went to the police and reported your suspicions, I wouldn't think that this would be actionable, otherwise nobody could report anything to the police for fear of being sued if they turned out to be wrong.
But if you wrote an article or posted something in a forum to the effect that the site was illegal, they may be able to sue over that. A lot may depend on what you thought was illegal about the site.