730 ILCS 150/5-10
The Illinois State Police shall mail a quarterly nonforwardable verification letter to each registered person who has been adjudicated to be sexually dangerous or is a sexually violent person and is later released, or found to be no longer sexually dangerous or no longer a sexually violent person and discharged, beginning 90 days from the date of his or her last registration. To any other person registered under this Article, the Illinois State Police shall mail an annual nonforwardable verification letter, beginning one year from the date of his or her last registration. A person required to register under this Article who is mailed a verification letter shall complete, sign, and return the enclosed verification form to the Illinois State Police postmarked within 10 days after the mailing date of the letter. A person's failure to return the verification form to the Illinois State Police within 10 days after the mailing date of the letter shall be considered a violation of this Article.
730 ILCS 150/5-10