Investigators from the Fair Trade Commission searched the offices of six machinery makers Thursday on suspicion of rigging bids for ventilation facilities for road tunnel projects funded by the government and the now-defunct state-run Metropolitan Expressway Public Corp.

The companies that were raided are Ishikawajima-Harima Heavy Industries Co., Ebara Corp., Mitsubishi Heavy Industries Ltd., Kawasaki Heavy Industries Ltd., Mitsui Miike Machinery Co. and DMW Corp.

The six machinery makers declined comment but confirmed that their offices were searched in the latest in a series of bid-rigging episodes related to public works.

The companies are suspected of rigging bids to keep order prices high and to share out contracts among them, in violation of the Antimonopoly Law.

The law carries penalties of up to three years in prison for individuals and a maximum fine of 5 million yen.

IHI, MHI and KHI were among more than 20 machinery makers whose offices were searched Tuesday on suspicion of rigging bids for floodgate projects financed by the government and local authorities and the state-run Japan Water Agency.