Japanese police on Tuesday raided the offices of the highway operator responsible for a tunnel collapse that killed nine people in central Japan. The fatal accident occurred on Sunday in the 4.7-km Sasago Tunnel on the Chuo Expressway, about 80km west of Tokyo, where three vehicles were buried by hundreds of collapsed concrete ceiling panels and caught fire. The tunnel, which links the capital to central Japan, was opened in 1977. Concrete slabs about 10 cm thick and weighing 1.2 tons fell from the ceiling of a 130-meter stretch in the aging expressway tunnel, according to the police and the highway operator Central Japan Expressway Co. The police searched six locations including the company\'s headquarters on suspicion of professional negligence leading to death and injuries. The highway operator said the accident may have caused by loosened bolts that hold the concrete panels to the tunnel\'s inner wall, adding that the bolts have not been replaced since the tunnel\'s opening 35 years ago as the firm has conducted only visual checks on the inner walls of the tunnel. It also said the workers did not detect any defects during a visual inspection in September. On Monday, the Land, Infrastructure, Transport and Tourism Ministry ordered expressway companies across the country to conduct emergency checks on the tunnels and submit reports on the checks by December 12.