Japanese police Tuesday raided a highway company's offices as part of a negligence probe over a weekend tunnel collapse that killed nine people, jangling nerves in a nation heavily reliant on its sprawling infrastructure.
The raids came as teams of inspectors fanned out across the nation to examine dozens of other tunnels of the same 1970s design.
At least eight officers from the Yamanashi Police Department entered the office of NEXCO in Hachioji, western Tokyo, a spokesman for the highway operator said.
"We are fully cooperating with the authorities over the accident," the spokesman told AFP.
Police also raided the company's office in Otsuki, Yamanashi, in connection with Sunday's tragedy at the Sasago tunnel, which passes through hills near Mount Fuji, Japanese media reported.
Officers were seizing documents relating to the firm's safety measures and inspection systems and were planning to question company officials on suspicion of professional negligence leading to death and injuries, Jiji Press said.
On Monday, the Japanese government ordered inspections of 49 highway tunnels as the focus of investigations at Sasago turned to decaying ceiling supports.
NEXCO said safety inspections consist largely of visual and acoustic surveys, with workers looking for cracks and other abnormalities in the concrete and metal parts.
Officials admitted that during the five-yearly check of the ceiling in September there had been no acoustic survey of the metal parts that support the panels, which each weigh up to 1.5 tonnes.
Three vehicles were buried when the concrete panels crashed down inside the tunnel, setting at least one vehicle ablaze and filling the tunnel with choking smoke.
Emergency workers retrieved nine bodies -- some of them charred by the fire. They included the body of a truck driver who reportedly telephoned a colleague immediately after the incident to ask for help.
Japan's booming economy in the 1960s and 1970s left a legacy of thousands of bridges, tunnels and other civil engineering projects.
Although building standards were generally considered to be fairly high, experts warn that these structures decay with age and many are now at the stage where they need serious maintenance work, or even replacement.