Deputies recovered multiple fake $100 bills Tuesday from students at Elkmont High School, according to a release from the Limestone County Sheriff’s Office.

On Sunday, deputies took a report about a student trying to pass a counterfeit $100 bill at the school. Investigators looked into the case Tuesday and recovered 20 more from students.

LCSO Public Information Officer Stephen Young said the bills were purchased online. He said it is legal to purchase and possess the bills until one of them is used in place of legal currency. At that point, even possession is against the law.

“Once you’ve used one, possession of more bills is illegal,” he said.

The sheriff’s office went on social media Tuesday night to remind the public they should check all bills before use to ensure their legitimacy. If an individual suspects they are in possession of counterfeit money, they should notify local authorities and at no point attempt to use the bills.

Common ways to determine if a bill is counterfeit include touch and the appearances of a watermark, color-shifting ink, a security thread and 3D security ribbon.

Legal paper currency is made of 25% linen and 75% cotton, with randomly disbursed red and blue fibers embedded. Bills in $10, $20, $50 and $100 denominations made in the 2004 style will change color from copper to green when tilted at a 45-degree angle. Those made in the 1996 style will change from green to black.

All legal currency except for $1 and $2 bills will have a clear thread denoting the denomination running through the bill. Finally, a watermark should be visible from either side of the bill when it is held up to a light.

Young said no charges had been made in the case at Elkmont as of Wednesday morning.