Three people were arrested on felony charges last week for their alleged involvement in a “substantial food stamp fraud ring.”

The scheme is responsible for stealing $4 million in purchases from Sam’s Club stores by illegally obtaining data from 8,000 electronic benefit transfers (EBT) cardholders, which are used by people who are involved in the Supplemental Nutrition Assistance Program (SNAP), the Michigan Attorney General’s office announced in a press conference on Tuesday, per The Detroit News.

Travis Newby, 39, Derriun Williams, 23, and Vanessa Williams, 47, were arraigned on Friday and face multiple counts of food stamp fraud and conducting a criminal enterprise. They could face up to 20 years in prison.

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The scheme targeted EBT cardholders (primarily in California), which were then reproduced in Michigan to make fraudulent purchases.

At this time, it’s unclear exactly how the card data was obtained, according to Eric Sterbis, an assistant attorney general, per The Detroit News.

The investigation is ongoing and more arrests are possible, Sterbis noted.

The investigation was initially launched after Sam’s Club and the Michigan Department of Health and Human Services reported large amounts of EBT purchases.

“Fraud is a problem impacting the retail industry,” said Claire Rushton, senior director of Walmart Global Investigations, in a statement to the outlet. “It’s only through collaborative efforts like this with states, communities and law enforcement that the issue can be addressed holistically.”

Sam’s Club is a division of Walmart.