Now federal authorities are investigating.
RELATED: Thief targets slot machine players at Rivers Casino in Des Plaines
Police say more than a dozen checks were stolen between December 12 and 23 after being mailed to employees.
The checks were worth between $300 and $1,600 each.
RELATED: Gambler arrested after allegedly caught on camera cheating at Rivers Casino in Des Plaines
The names and amounts on the checks were altered and were then cashed at currency exchanges and other businesses in Chicago.