ALTOONA, Pa. -- Twelve $100 bills have changed hands so far in Blair County. On Monday, police said the worst part is it's hard to tell that they are fake.

The $100 bills don't show up as fakes with a counterfeit pen because they are real $5 bills that have been bleached and reprinted.

Last week, a Kmart clerk thought something seemed amiss when a customer bought about $2 worth of merchandise with one of those bills and pocketed the change. Other Altoona area stores alerted police when they realized they had also been scammed.

On Monday, police alerted businesses of what they should look for to make sure they do not get scammed.

Police said when you hold a $100 bill up to the light, you should see a Benjamin Franklin watermark on the right side. The fakes have Abraham Lincoln instead. Also, if the bill smells like laundry detergent or dryer sheets, it could be one of the counterfeits.

Police are working to track down the people who started the scam. They are also contacting the Secret Service.