I hope this message finds you well, present circumstances excluded. The reason the authorities called you about this situation is because they are required to under Illinois law. The law is called the "Rights Of Crime Victims & Witnesses Act". Here is a link to the law...which is quite long: http://www.ilga.gov/legislation/ilcs/ilcs3.asp?ActID=1970&ChapAct=725%26nbsp%3BILCS%26nbsp%3B120%2F&ChapterID=54&ChapterName=CRIMINAL+PROCEDURE&ActName=Rights+of+Crime+Victims+and+Witnesses+Act%2E
The Attorney General's Office is also obligated to contact you when the person is up for parole and when they are about to be released from prison under this law.
As to why they would contact you and then seem to side with the perpetrator, I have no idea. That is very counterintuitive. Moreover, it sounds like he was convicted of a felony based on the information you provided with him being in prison. As such, and as a convicted felon, he cannot possess or be around a firearm of any type.
To that point, guns used to commit a crime and later confiscated by the state are supposed to be destroyed by the state.
You need to write a letter to the Attorney General's office particularly pointing these facts out. You also need to send a copy of that letter to your State Representative and Senator and request assistance in making sure that this individual does not receive that which he has requested.
Make sure that you keep copies of all letters sent to these people for your own records. If it appears that they are going to give this property back, you could petition the court that convicted this person to issue an injunction that would prevent the person from receiving this property.
Let me know if you have any other questions or comments.
Best wishes going forward!