NEW YORK (CNN) - An attorney representing nine undocumented immigrants who worked as janitors at Wal-Mart said Monday he has filed suit against the retail giant, charging it and its contractors with discrimination.

Attorney Gilbert Garcia said the suit alleges Wal-Mart, the world's biggest retailer, knew the workers were undocumented and, as a result, paid them at lower wage rates, did not pay overtime, and failed to make workers' compensation payments.

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Wal-Mart has denied hiring illegal workers. The company said the cleaning crews were hired by contractors, and that it requires its contract firms to follow the law and hire only legal workers.

Asked specifically about the suit, company spokeswoman Sharon Weber said the company is reviewing the illegal immigrants' complaint and will not comment beyond saying it denies the allegations.

Federal prosecutors have informed the company that it is under investigation by a grand jury, Wal-Mart officials said.

On Oct. 23, federal agents arrested more than 200 workers who clean Wal-Mart stores, alleging they were undocumented workers.

Garcia said the workers were caught up in the Oct. 23 sweep and face possible deportation.

Wal-Mart (WMT: down $0.16 to $58.26, Research, Estimates) stock was little changed in midday trading on the New York Stock Exchange.