An attorney or anybody else should not threaten anybody with the prosepect of criminal prosection if you don't ante up. In fact this could be a violation of the Rules of Professional Conduct. For example, if an attorney wrote another attorney with a letter like the one you received he could be sanctioned by the state bar association for threating legal or administration reprisals if you don't settle with him.  It isn't the landlord who makes the determination whether to prosecute your or not, it's the People of Illinois. There are many procedural details that the government must do before filing a criminal complaint against you; and if it's a misdemeanor the offense must be committed in the presence of a police officer or the "victim" must make a citizen's arrest.  If it's a felony an arrest warrant must be issued by a magistrate or judge based on the government's representation that you are guilty of a felony. The important thing now is not to discuss this case with the police or anyone else except an attorney.  It cannot help you.  Nobody gets convicted because they exercised their constitutional right to remain silent--the Fifth Amendment. I think a letter to the attorney who wrote the letter and citing the applicable rule of professional conduct will shut him up.  It is almost like extortion or blackmail.

I hope this information is helpful to you.