Wow, I'm so sorry to hear this, A.R. As infuriating as this is, don't be discouraged - it's part of business and part of the learning process.
I think Erik and Brie had great answers. I would like to add that "Good Documentation Practices" (GDP) are SUPER CRITICAL when stuff like this happens. I don't know if this is the right way, but here's what I would do...
1. Check and gather your lease agreements, any receipts for the security, and any other pertinent documents handy.
1a. Also get anything you can regarding late payment records or other discrepancies that occurred during the time that this tenant occupied the property. Any emails that were sent, text messages, etc., are valuable here. Assemble them into pdf's or old-fashioned paper copies. Organize them in such a way so they're easy to explain and for anyone to understand.
2. Find an attorney that specializes in landlord/tenant disputes ASAP. Even one that works mostly with tenants should be familiar with the laws. Show him all this info and see what can be done.
You should also educate yourself on the laws in your area. If Chicago is anything like NYC, the laws lean more on the tenants' side in order to combat the growing homelessness prevalent in big cities. Sadly, this often comes at the landlord's expense.
Good luck!
