DVD-rental vending machine maker RedBox today warned customers to be on the lookout for any unusual activity or physical changes to local RedBox kiosks, after the company discovered evidence that criminals had retrofitted at least three of the machines with devices to steal credit-card information. An example of a RedBox machine with an illegal credit card skimmer attached. The company said several RedBox machines had been fitted with "skimmers" -- magnetic stripe reading and storage devices that can be installed over the top of existing card readers. RedBox said it found an illegal skimming device attached to one machine in Tempe, Ariz., and that it had discovered evidence of skimming at two other locations in Las Cruces, N.M. In a notice posted on its Web site, Redbox said is not aware of any fraudulent activity or transactions using its customers' accounts, and that it is working to minimize the risk of this happening. But the company is urging customers to be vigilant for signs of tampering at any of its 7,400 Redbox locations nationwide. An example of an approved RedBox reader. Customers who suspect their local Redbox may have been tampered with should contact 630-756-8866, e-mail alerts AT redbox.com or notify the manager of the store or restaurant that houses the machine. Criminals use credit-card skimming devices to store data that can be used later for identity theft. Typically, bank ATM machines are the target of such scams. In an incident last April, a bank in Tysons Corner, Va., warned customers that thieves had installed a skimmer at an ATM, as well as a wireless camera to visually record the 4-digit PINs customers entered to withdraw cash from the machines.