Photo/IllutrationA temporary bridge girder is moved into position at Tokyo's Shinagawa Station where a new facility will be constructed for the Linear Chuo Shinkansen Line. (Asahi Shimbun file photo)

Prosecutors are set to throw the book at big-name contractors suspected of bid-rigging for a mammoth super-fast maglev train line project.

Sources said prosecutors are preparing to invoke the Anti-Monopoly Law over work done for the Linear Chuo Shinkansen Line, such as new facilities at Tokyo's Shinagawa Station.

The total cost of the maglev project has been estimated at 9 trillion yen ($84.1 billion).

Four "super general contractors" have been questioned by prosecutors on a voluntary basis, but only two companies, Obayashi Corp. and Shimizu Corp., have acknowledged wrongdoing, sources said.

They threw themselves at the mercy of the Fair Trade Commission's system of leniency in the hope of winning reduced fines.

The sources said that a former executive of Taisei Corp. as well as the department chief at Kajima Corp. in charge of the maglev project continue to insist there was no collusion.

To support the contention, the Taisei former executive pointed to the fact the company failed to win the contract for construction of a new Nagoya Station that it wanted.

The official was quoted as telling investigators, "We should have been awarded the contract if collusion had actually taken place."

The Kajima official denied having had the authority to make any decision about which company received which contract.

However, the general interpretation of the Anti-Monopoly Law is that collusion occurs once companies involved in a project agree to coordinate how contracts are doled out. Whether or not a company wins the contract it initially wanted is not directly relevant for making a case for collusion.

Prosecutors are apparently planning to use the admissions by Obayashi's former vice president and Shimizu's former executive to make their case.

According to sources, the collusion on the Shinagawa Station and other projects effectively began when the former Taisei executive contacted officials from the three other companies.

After Central Japan Railway Co. (JR Tokai) decided in around 2014 how the construction project would be divided up, the four companies began discussions on who would win what.

The former Taisei executive and the former vice president of Obayashi took the lead in convincing the officials of the two other companies to go along.

According to JR Tokai officials, they designated which companies could submit bids for the Shinagawa Station project. The four super general contractors were believed to be among those who were designated.

In September 2015, a joint venture centered on Shimizu won the contract for the northern construction area and in the following month a joint venture headed by Obayashi won the contract for the southern portion.

In September 2016, another joint venture headed by Obayashi won a contract for part of the work at Nagoya Station.