Japanese authorities are investigating subcontractors on suspicion they forced workers at the tsunami-hit nuclear plant to underreport their dosimeter readings so they could stay on the job longer.

Labor officials said Sunday that an investigation began over the weekend following media reports of a cover-up at the Fukushima Dai-ichi plant, which suffered multiple meltdowns following last year’s earthquake and tsunami disasters.

One of the subcontractors of Tokyo Electric Power Co., which operates the plant, acknowledged having nine workers cover their dosimeters with lead plates late last year.

It’s the first time the government is looking into the case, believed to be part of a widespread practice at the plant following the disaster.

The government raised the emergency exposure limit soon after the accident but lowered it in late November.