Twenty Walmart workers sued the company and two Chicago area staffing agencies, Labor Ready Midwest and QPS Employment Group, for alleged failure to pay overtime and less than the minimum wage on Monday. The workers, who mainly stock shelves at Walmart stores, want backpay from the last four years.

A Walmart spokesman told the Huffington Post that the United Food and Commercial Workers is driving the lawsuit. Walmart says UFCW seeks publicity and the ability to unionize the workers.

The lawsuit comes after subcontractors in a Will County Walmart warehouse last month sued RoadLink, another employment agency that does business with Walmart, for similar wage theft issues. The legal action also follows Walmart retail workers across the country staging walkouts earlier this month.