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PHILADELPHIA (CBS) — The owner of a South Philadelphia 7-Eleven says he’s seen a surge in bogus bills from customers – ranging from fake $20 to $100 bills. The Secret Service is investigating the counterfeit cash that is making rounds in the region.

Spotting fake cash is vital for South Philadelphia owner Vincent Emmanuel. He has owned the 7-Eleven store at Passyunk and 23rd Street since 1981.

“This is from probably in the last couple of months. All of a sudden, we see a surge in counterfeit dollar bills,” said Emmanuel.

The bank notified Emmanuel of the bogus bills after his staff took fake $20 and $100 bills from customers. The 7-Eleven has a system they use to detect fake money.

“It just won’t take it because it knows it’s fake,” said Emmanuel.

However, one counterfeit detecting technique may not be as reliable as the use of a marker.

“The problem is that somebody can bleach the $1 bill and you put the $20 on top of that, it’s undetectable,” explained Emmanuel.

Assistant special agent in charge Al Feaster is one of about 25 secret service agents in Philadelphia investigating financial crimes.

“We’re aware of this and we’re actively and aggressively investigating,” said Feaster.

Feaster says searching for color changes, watermarks, and fine print are ways to identify real bills.

“I’m looking for micro-printing and I’m also going to take a look here for the red and blue fibers,” he said.

If caught, criminals could face federal charges of counterfeit and manufacturing. Emmanuel says the criminals in his store are young.

“They don’t even understand how serious of a crime they’re doing,” he said.