The Georgia Department of Corrections (GDC) said the underpayments occurred because prison officials misinterpreted a state agency rule that officers may work no more than 12-hours on an overtime shift.

“There was never, and will not be a time when the GDC would make any deliberate attempt to deprive them of compensation for their hard work and commitment,” it said in a written statement.

The department did not make officials available for an interview. It responded to written questions through a spokeswoman.

Impacted officers received nearly $380,000 in back pay. State Department of Corrections job ads list starting salaries as low as $28,000 a year.

Problems with prison safety and staffing have taken center stage in the past 18 months with an ongoing national prison strike and the June 2017 killing of two Georgia corrections officers in Putnam County during an inmate escape. Records showed one of them worked extra days almost every week for at least three months before he was killed.

The corrections department has known about its staffing issues for years. A 2014 audit showed turnover costs the state tens of millions of dollars. The department's two-year retention rate still hovers at 42 percent, according to a 2017 state report. This means that more than half of new hires leave within two years.

Chronic staffing shortages are especially difficult to solve in rural areas, Geraghty said. The officer vacancy rate stood at 16.29 percent for fiscal year 2018, department spokeswoman Joan Heath said.

The AJC discovered that officers went unpaid while reviewing a database of legal settlements made by state government. It obtained the federal labor department records through a federal Freedom of Information Act request.

The labor department launched its first investigation in February 2017 at Smith State Prison. It found that the prison, which shorted 232 officers on pay over two years, failed to pay workers for required training. When officers did get paid for overtime work, they received straight time rather than time-and-a-half, the investigation found. This is a violation of the Federal Labor Standards Act.

The prisons also failed to keep proper employee records, the labor department reports said. Investigations at Coastal and Valdosta state prisons followed.

After releasing their findings, labor department investigators walked the wardens and their supervisors through federal rules in detail, reports state. At Smith, where 232 workers were shorted on pay, investigators told Warden Douglas William that waiting time, certain travel time, training time and safety meetings were work hours and had to be paid.

“He was advised that ‘off the clock’ work should never occur,” the report stated.

Data reporter Jennifer Peebles contributed to this report.