A fraud investigation targeting Obayashi Corp. is spreading to the nation’s other major general contractors as prosecutors scrutinize deals related to the ¥9 trillion maglev project, sources said Monday.

Those being questioned on a voluntary basis include workers from multiple construction firms, including leading contractor Kajima Corp., sources close to the case said. Obayashi and Kajima are two of the elite group of five dubbed “super-general contractors” in Japan.

Obayashi is being probed by Tokyo prosecutors for a contract it won jointly with Toda Corp. and JR Tokai Construction Co. in April 2016 to build an emergency exit by September 2019 for the maglev station planned in Nagoya, the sources said.

The maglev project will link Tokyo, Nagoya and Osaka through a next-generation shinkansen line for magnetically levitated trains traveling as fast as 500 kph.

The contract in question is one of four Obayashi won from Central Japan Railway Co. (JR Tokai), with its joint venture partners from October 2015.

The prosecutors suspect Obayashi obstructed JR Tokai’s bidding process by making separate pricing arrangements with other contractors in the maglev project. Obayashi’s headquarters in Tokyo and other sites were searched on Friday and Saturday by prosecutors.

JR Tokai has signed construction deals for 22 projects covering tunnels, stations and emergency exits related to the next-generation bullet train, which is supposed to link Tokyo with Osaka by 2045.

Services on the 286-km stretch from Tokyo to Nagoya are slated to begin as early as 2027, slashing the 88-minute trip to just 40 minutes.

Other major participants in the maglev project include Shimizu Corp. and Taisei Corp.

The prosecutors have already questioned Obayashi’s executive vice president, who oversees the civil engineering unit, and an official at its Nagoya office, according to the sources.

The executive vice president, 66, and the other officials questioned have denied involvement in the allegedly fraudulent acts, saying they did not make advance arrangements, the sources said.

The value of the contract in question has not been disclosed. JR Tokai Construction is wholly owned by JR Tokai, which runs the Tokaido Shinkansen Line linking Tokyo and Shin-Osaka stations.

“We will fully cooperate” with investigators, an Obayashi spokesman said.

JR Tokai said it is not aware of any fraudulent behavior in its bidding process.

Services on a 286-km stretch between Tokyo and Nagoya are slated to begin as early as 2027, whisking passengers to the capital of Aichi Prefecture in only 40 minutes, less than half the 88 minutes it takes on the quickest shinkansen today.

The next generation high-speed train, the world’s first using super-conducting magnetic levitation technology, will travel at a top speed of 500 kph (310 mph).