Investigators from the Fair Trade Commission searched some 30 firms Thursday in connection with alleged bid-rigging for public works projects commissioned by the city of Niigata.
The move followed a similar raid on Niigata City Hall on Wednesday on suspicion city officials were involved in the case.
The antimonopoly watchdog also searched the offices of some 30 construction companies, including Tobishima Corp. and Hazama Corp. on Wednesday.
A day earlier they searched about 40 other firms.
Thursday's raids brought the total to more than 100.
According to investigative sources, the construction companies held talks before tenders were opened for civil engineering projects by the Niigata Municipal Government to determine which companies should be awarded the contract and for how much.
City officials may have been involved in the scam through such means as leaking information on the projected prices for the projects, the sources said.
The city government orders more than 10 billion yen worth of civil engineering works each year, municipal officials said.
A law was put into force in January to prevent national and local government employees from having a hand in bid-rigging.
The city has set up its own investigation team to look into the matter.