Major Japanese construction company Obayashi Corp. has admitted to an antitrust violation over bidding for work linked to the Tokyo-Osaka maglev train link, sources close to the matter said Tuesday.

Obayashi has admitted to the Japan Fair Trade Commission that it was involved in bid obstruction with three other major contractors -- Taisei Corp., Kajima Corp. and Shimizu Corp. -- exempting it from a fine under the commission's leniency rules.

Meanwhile, the Tokyo District Public Prosecutors Office and the FTC on Tuesday searched the headquarters of Obayashi and Taisei over their alleged involvement in the antitrust breach.

Tokyo prosecutors have been looking into how Japan's four biggest contractors won 15 out of the 22 construction tenders put out by Central Japan Railway Co. for the 9 trillion yen ($80 billion) project. The maglev line will connect Tokyo and Osaka, about 500 kilometers west of the capital, in about an hour.

The revelation came a day after prosecutors searched the headquarters of Kajima and Shimizu in Tokyo over the maglev train bid, suspecting them of conspiring with Obayashi and Taisei to determine which contractor will win orders.

Earlier this month, the prosecutors' special investigation squad raided Obayashi's head office, suspecting it engaged in fraudulent obstruction of business in a tender to build an emergency exit for the maglev train line in Nagoya.

Executives of the four leading construction companies have also been quizzed on a voluntary basis.

Following the initial raid, Obayashi reported to the FTC that it has conspired with the three construction companies for tenders for maglev-related construction, the sources said.

But all the four contractors declined to comment Tuesday on Obayashi's report to the FTC while investigations on the matter are under way.

Under the FTC's leniency rules, companies voluntarily reporting antitrust violations escape a fine. The company that does so first will also avoid criminal charges.

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 and extend the line to Osaka by 2037. The government has provided a total of 3 trillion yen to JR Central for the project.