YULEE, Fla. -

Nassau County Sheriff Bill Leeper is warning residents, business owners and employees to be on the lookout for counterfeit currency being circulated recently.

Since Aug. 29, deputies have received reports of counterfeit bills in $20 and $50 denominations that are being passed as payments to local businesses.

Some of the men caught on store video cameras were last seen driving a silver Grand Am-style vehicle and a green Dodge truck.

"It's a federal offense and you're facing some serious charges with that and some federal time once conviction," Channel 4 crime analyst Ken Jefferson said.

Deputies are encouraging residents to closely examine currency by looking carefully at the money they receive. Residents should be aware of the security features located on currency and check the serial numbers if multiple bills are received at once.

The three main security features located on bills are: Shifting inks on the dollar amount located on the face side in the bottom right corner; security threads, which display the denomination of the currency embedded in the bill; and the watermark located on the face of the bill on the right side.

"These bills look very genuine at first glance and will pass the 'pen test' vendors often use," Leeper said. "This is because the bill is printed on a $1 bill. It is common for those involved in these crimes to bleach the $1 bill and then print larger denominations onto the now-blank bill."

In addition, the Sheriff's Office is offering these tips to detect suspicious or counterfeit currency: The genuine portrait appears lifelike and stands out distinctly from the background. The counterfeit portrait is usually lifeless and flat. Details merge into the background, which is often too dark or mottled.

On a genuine bill, the saw-tooth points of the Federal Reserve and Treasury seals are clear, distinct and sharp. The counterfeit seals may have uneven, blunt or broken saw-tooth points.

Genuine serial numbers have a distinctive style and are evenly spaced. The serial numbers are printed in the same ink color as the Treasury Seal. On a counterfeit, the serial numbers may differ in color or shade of ink from the Treasury seal. The numbers may not be uniformly spaced or aligned.

Deputies are asking that residents and businesses that locate suspicious currency report the incident to deputies or call the Sheriff's Office at 904-225-5174. Anyone with information about the identity of the man pictured above in surveillance video from one store is asked to call the Sheriff's Office or Crime Stoppers at 866-845-TIPS.

For further information on detecting counterfeit U.S. currency, residents are encouraged to visit the United States Secret Service website.