Published: Apr 21, 2011 9:57 PM EDT
Updated: Apr 21, 2011 7:03 PM EDT

DENVER (AP) - Federal prosecutors are investigating Chipotle Mexican Grill over issues with documents showing whether its employees can work legally in the U.S.

The Denver-based restaurant chain last year fired about 450 Minnesota employees who couldn't prove they were eligible to work in the U.S. The dismissals came after Immigration and Customs Enforcement officials raised questions about the validity of employees' work authorization documents.

In December, Homeland Security officials requested worker authorization documents for employees in Virginia and the nation's capital.

Chipotle disclosed in a regulatory filing Thursday that the office of the U.S. Attorney for the District of Columbia sent it a notice this month that it also is investigating the matter through its criminal division.

The company contends it is complying with the law.

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