Investigators on Tuesday raided the Japan Railway Construction, Transport and Technology Agency as well as two private firms over alleged bid-rigging for snow-melting equipment on the Hokuriku Shinkansen Line, investigative sources said.

Around 30 investigators from the Tokyo prosecutor's office and the Fair Trade Commission searched the agency's Tokyo branch, while others inspected its main office in Yokohama and equipment manufacturers Nippon Air Technologies Co. and Tonets Corp., both based in Tokyo.

Employees of the companies declined comment.

The Tokyo branch opened bidding for 13 contracts worth ¥26 billion to install snow-melting panels and hot water sprinklers along bullet train tracks between March 2011 and November 2012.

More than 10 companies allegedly colluded to determine bid prices and contract winners beforehand, the sources said.

Bidding records indicate 10 of the winning bid prices were higher than 90 percent of agency-planned prices, including five that exceeded 99 percent of the planned prices.

An official of the independent administrative agency is suspected of leaking the planned prices for the contracts to the companies.

Most agency workers and company employees questioned by the JFTC have admitted to their involvement in the bid-rigging, the sources said.

The construction contracts covered the Hokuriku Shinkansen Line section from Nagano to Kanazawa, which is to open by the end of March 2015.