A Riverhead woman who worked as a Walmart cashier pretended to ring up sales but was part of a scam in which she let pretend customers leave with nearly $11,000 in merchandise over a month’s time, Riverhead Town police said Tuesday.

Bianca N. Roberson, 24, was charged with third-degree grand larceny Jan. 26 and arraigned after Walmart’s asset protection team called police with video and store receipt evidence, police said.

In more than 20 cases between May and Roberson’s firing in June, friends or acquaintances would come by with carts full of goods, but she would charge them only for something minor, such as a soda, and do price checks on the other items, Riverhead detectives said.

Often, the fake shoppers would leave the store with more than $1,000 in unpaid merchandise, from stereos to outdoor furniture, police said.

She hadn’t been working at the Riverhead Walmart for long, and it was unclear when the scam started, police said.

But store managers became suspicious in June after they spotted a man who appeared to be a shoplifter because he was piling his cart high with items without looking at prices, police said.

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Managers were surprised when he went to pay — at Roberson’s checkout aisle, police said.

In reviewing the electronic sales records, store managers saw she rang up one item, investigators said. The video showed Roberson talking to him in the parking lot later, police said.

When Walmart loss prevention experts looked at video going a month back, they saw the trend at Roberson’s checkouts and saw her talking to the fake shoppers in the parking lot, detectives said.

It appeared at least 10 different people pretended to be customers, police said, and detectives are looking for Roberson’s accomplices. Police said Walmart lost more than $10,782 in goods.

Detectives ask anyone with information to call them at 631-727-4500.