Ukraine’s top anti-corruption authorities on Monday searched properties linked to senior government officials and a former business partner of President Volodymyr Zelenskyy as part of a sweeping investigation into alleged graft in the country’s energy sector.

News of the probe came amid escalating Russian aerial assaults on Ukraine’s energy infrastructure that have led to rolling blackouts and growing anger at Ukrainian authorities for failing to protect power facilities.

The alleged scheme involved “kickbacks” from contractors hired to build fortifications to defend energy infrastructure against Russian missiles and drones. It will add to concerns among Ukraine’s chief partners that the country needs to tackle graft as it seeks billions more dollars in western support for its war effort and economy.