Japanese authorities are investigating subcontractors on suspicion that they forced workers at the tsunami-hit nuclear plant to underreport the amount of radiation they were exposed to so they could stay on the job longer.

Labor officials said Sunday that an investigation had begun over the weekend following media reports of a cover-up at the Fukushima Dai-ichi plant, which suffered multiple meltdowns following the March 2011 earthquake and tsunami disasters.

A subcontractor of plant operator Tokyo Electric Power Co., or TEPCO, acknowledged having nine workers cover their dosimeters with lead plates late last year so the instrument would indicate a lower level of radiation exposure.

The investigation marks the first time the government has looked into the case, believed to be part of a widespread practice at the plant since it was hit by the worst atomic crisis since Chernobyl....