Police on Tuesday searched the offices of the firm operating the Chuo Expressway, the site of tunnel collapse in Yamanashi Prefecture that left nine people dead Sunday.
Authorities, in the presence of construction experts, also conducted an on-the-spot inspection inside the collapsed Sasago Tunnel about 80 km west of Tokyo.
The deadly incident occurred in the tunnel Sunday morning, when several ceiling panels, each weighing about 1.2 tons, suddenly fell onto moving vehicles.
The collapsed slabs were suspended by metal rods fixed to the inner wall of the tunnel. Operator Central Nippon Expressway Co. has admitted that some rods came loose and that the accident could have been caused by aging bolts, concrete panels and adhesives.
Suspecting professional negligence, police searched six locations, including the company's head office in Nagoya and its Hachioji branch in Tokyo, in addition to Central Nippon Expressway Engineering Tokyo Co., which conducted an inspection of the tunnel in September that found no anomalies.
The police are expected open a probe into whether safety measures taken by Central Nippon Expressway were appropriate. They will also analyze images recorded on surveillance cameras installed inside the tunnel.
The tunnel has not undergone any major repairs of its ceiling since its opening in 1977.