The Grove Police Department is warning businesses in eastern Oklahoma to be on the lookout for counterfeit 20- and 100-dollar bills, after some turned up over the past week.
Police Chief Mark Morris says criminals have "washed" the bills, converting one-dollar and 5-dollar bills into 20s and 100s.
He says the bills pass the ink test, but if security strip inside the bills still shows the original denomination.
Morris says his department is working with the Secret Service in the investigation.
He says they've received at least four reports of counterfeit bills in the past week.
Anyone who believes they have received a counterfeit bill or have information on this case should contact the Grove Police Department at 918-786-6121.