Tokyo:Japanese police Tuesday raided the offices of a highway company in a negligence probe over a deadly weekend tunnel collapse that raised questions over the safety of the nation's ageing infrastructure.
The raid came as teams of inspectors fanned out across the country to examine dozens of other tunnels of the same 1970s design, part of an extensive network that are vital transport arteries in the mountainous country.
Television footage showed more than a dozen police officers entering the headquarters of NEXCO in Nagoya, central Japan, with the company's maintenance and safety records being targeted.
Employees are expected ultimately to be quizzed on suspicion of professional negligence leading to death and injuries, local media said, although no arrests have been made.
Police also raided NEXCO offices in Tokyo and eastern Yamanashi prefecture, a company spokesman said, in connection with Sunday's accident at the Sasago tunnel which passes through hills near Mount Fuji.
"We are fully cooperating with the authorities over the accident," the spokesman told AFP.
Separately, officers launched an on-site investigation at the collapsed tunnel some 80 kilometres (50 miles) west of Tokyo. Television footage showed police vehicles and a ladder-truck going into its mouth.
Members of the government's accident investigation commission were also scheduled to visit the nearly five-kilometre (three-mile) structure later in the day to begin their initial probe, officials 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 of them 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.
NEXCO said it was still unknown when the firm will be able to reopen the tunnel on the Chuo highway, a major artery used by around 47,000 vehicles a day.
Heavy traffic clogged roads being used as a bypass of the section on Tuesday, media reports said, adding that the accident has started affecting the flow of goods between Tokyo and the country's west.
Japan's extensive highway network criss-crosses the mountainous country, with more than 1,500 tunnels. Around a quarter of these are more than 30 years old, according to the Transport Ministry.
The country is also prone to earthquakes and despite a tightening of safety regulations over the last 20 years, older structures could be vulnerable to the regular movements, experts have warned.
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 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.