If you're in Missouri, go to page 11 of this:
https://www.ago.mo.gov/docs/default-...-tenantlaw.pdf
You can sue to recover up to twice the amount unlawfully withheld.
Your statutory citation is Section 535.300 of the Missouri Revised Statutes.
https://law.justia.com/codes/missour...ction-535.300/
At 2x the security deposit, but no recovery of attorneys fees (very much disfavored in the US under nearly all circumstances), you're almost certainly bringing a small claims action, and would NOT hire an attorney at any point along the way. The lawyer will eat up your 2x security deposit quicker than you can say "What the ****? These guys are EXPENSIVE!"
So that's it. Call them and explain it, or else file a small claims action.