A Mississippi police department in one of the nation’s poorest counties unconstitutionally jailed people for unpaid fines without first assessing whether they could afford to pay them, the U.S. Department of Justice said Thursday.

The announcement comes amid a Justice Department probe into alleged civil rights violations by police in Lexington, Mississippi. The ongoing investigation, which began in November, is focused on accusations of systemic police abuses in the majority-Black city of about 1,600 people some 65 miles (100 kilometers) north of the capital of Jackson.

In a letter addressed to Katherine Barrett Riley, the attorney for the city of Lexington, federal prosecutors said the Lexington Police Department imprisons people for outstanding fines without determining whether the person has the means to pay them — a practice that violates the Fourteenth Amendment. Riley did not immediately respond to a phone message Thursday.