Tokyo prosecutors have raided the headquarters of at least two of Japan’s biggest construction firms for alleged antitrust violations linked to $80 billion worth of magnetic levitation (maglev) train line projects.

Shimizu Corp (1803.T) and Kajima Corp (1812.T) were raided early on Monday morning, spokesmen for the two companies separately said.

Another construction firm, Obayashi Corp (1802.T), is already under investigation for suspected bid-rigging over maglev-related contracts.

A spokeswoman for the Tokyo District Public Prosecutor’s Office said it could not comment on individual cases.

The other member of the so-called “big four” group of Japanese construction firms involved in the maglev project, Taisei Corp (1801.T), had not been raided, a spokesman said.

Kajima shares plunged as much as 4.6 percent and Shimizu shares fell as much as 3.7 percent on news of the raids. Taisei shares were also down 3.3 percent on media reports it would be raided next. The Tokyo Stock Exchange was 1 percent higher.

The investigation relates to a 9 trillion yen ($80 billion) maglev line that is under construction linking Tokyo, Nagoya and Osaka.

Trains running at speeds of up to 500 km (311 miles) per hour, through tunnels deep under mountainous terrain, will cut train travel time between Tokyo and Nagoya to 40 minutes from 100 minutes or more when operation begins in 2027.

The big four contractors have won roughly an equal number of orders for about 70 percent of the projects, according to Japanese media reports.