Prosecutors on Monday raided the headquarters of leading contractors Kajima Corp. and Shimizu Corp. in Tokyo over an alleged antitrust violation in connection with maglev train construction tenders.

Kajima and Shimizu are among the four major construction companies that won contracts to build the ultra high-speed train line that will connect Tokyo and Osaka, about 500 kilometers west of the capital, in about an hour.

The companies are suspected of conspiring to determine which contractor will win orders ahead of the bidding. The special investigation squad of the Tokyo District Public Prosecutors Office is looking into the case together with the Japan Fair Trade Commission.

According to Central Japan Railway Co., the operator of the 9 trillion yen ($80 billion) maglev train project, Kajima and Shimizu, plus the two other major contractors Obayashi Corp. and Taisei Corp., won contracts for 15 out of the 22 construction tenders put out.

Obayashi and Taisei are also expected to be searched by investigators in the near future.

Both Kajima and Shimizu said they will cooperate with investigations. Kajima said four other locations, including a civil engineering branch office adjacent to its headquarters, were also searched.

Earlier this month, the prosecutors raided the headquarters of Obayashi for an alleged fraudulent obstruction of business in the tender process that resulted in an Obayashi-led consortium winning a 9 billion yen contract in April 2016 to build an emergency exit on the line in Nagoya.

After questioning executives of the four major construction firms on a voluntary basis and analyzing confiscated materials, the prosecutors apparently came to suspect the companies violated antitrust law by repeatedly pre-negotiating potential tender winners and bid prices.

Obayashi, Shimizu and Taisei won four contracts each while Kajima secured three to build separate sections of the maglev train project.

When finished, JR Central will operate the world's first train that uses superconducting magnetic levitation technology. It will travel at a top speed of 500 kilometers per hour, much faster than current shinkansen bullet trains.

JR Central aims to start the new high-speed train services between Tokyo and Nagoya in 2027. The Japanese government has provided a total of 3 trillion yen to the railway company in a bid to extend the line to Osaka by 2037.