Officials from the Japan Fair Trade Commission are investigating the actions of some of the country's biggest energy providers.
They suspect the companies colluded in a way that undermined competition and unfairly impacted consumers.
The officials are probing Kyushu Electric and its subsidiary Kyuden Mirai Energy, plus Kansai Electric and Chugoku Electric. Sources say the four allegedly struck a deal to stay off each other's turf, which meant they would not target each other's customers.
Competition across Japan's retail electricity market has become increasingly fierce since the sector was fully liberalized in 2016.
Sources claim the power companies formed their cartel around 2018.
The firms told NHK they are cooperating with the investigation.
Fair Trade Commission officials searched the offices of Chubu Electric, Kansai Electric and Chugoku Electric earlier this year on suspicion of forming a similar cartel.
