The U.S. Secret Service says about half of its current counterfeit cases in the Washington area involve using real currency to make phony bills.

Special agent in charge Todd Kreisher says counterfeiters bleach $5 bills and reprint them as $100 bills.

Kreisher says the fake money looks real, and the bills aren't detected by some machines because they are printed on real currency paper. Holding them to the light, however, reveals that the watermark has Abraham's Lincoln's image, not Benjamin Franklin's.

Kreisher says arrests have been made in D.C., Prince George's County, Montgomery County and Virginia.

The Secret Service issued an alert Monday to area law enforcement agencies to warn businesses about the fake bills.