It wouldn't be too hard to phrase it in such a way that it seems completely legitimate. For example, pull one of the Russian Mafia guys in and offer him a deal

It wouldn't be too hard to phrase it in such a way that it seems completely legitimate. For example, pull one of the Russian Mafia guys in and offer him a deal to inform, but tell him it's a limited time offer "because we'll have this guy soon and he'll squeal". (Also even if there are hints of illegality juries are generally unlikely to convict cops).