BOSTON -- The new $20 bills have features to make them nearly impossible to counterfeit, but police south of Boston say fake bills have already made it into circulation. Now, the Secret Service is helping in an investigation.

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In Brockton, Mass., the phony $20s have been located at Petti's Market, George's Cafe and a Radio Shack. In Bridgewater, Mass., they've shown up at the lunchroom of the Bridgewater-Raynham Regional High School and a McDonald's.

Treasury officials said the $20 bill is the most counterfeited bill, which is why it came out with a new design featuring a blue eagle on the front, along with shades of bluish-green and peach ink.

"We've added these subtle background tints that give us the ability to provide color in the note while still maintaning it as a very traditional American note," said Tom Ferguson of the U.S. Bureau of Engraving.

The new $20s also have a new shield and the words, "Twenty USA". The oval around Andrew Jackson's head is gone, and on the back, if you look closely, you'll see tiny yellow 20s. The security thread on the left side of the bill, and the watermark on the right side, can be seen if the bill is held up to the light.

"The new design makes traditional counterfeiting more difficult," said W. Ralph Basham of the U.S. Secret Service.

Officials encourage consumers and business owners to check their money closely for the new security features to ensure their authenticity.

The fake bills showing up in Massachusetts do not have the security thread or watermark.