required in the motorway network, parts of which including the accident site were built during the economic boom of the 1960s and 1970s.
"As a major factor, we suspect ageing," an official from highway operator Nexco said, referring to the tragedy at the Sasago tunnel which passes through hills near Mount Fuji 80 kilometres (50 miles) west of Tokyo.
Engineers on Monday began inspections at three other tunnels in the region with the same design, as well as the Sasago carriageway. There are around 20 such tunnels nationwide, reports said.
Footage from inside the tunnel showed concrete panels had collapsed in a V-shape, possibly indicating some kind of weakness in the central supporting pillars suspended from the roof, experts said.
Nexco said safety inspections consist largely of visual surveys, with workers looking for cracks and other abnormalities, or listening to the acoustics of the concrete and metal parts by hitting them with hammers.
Officials admitted that during the five-yearly check of the ceiling in September there had been no acoustic survey of the metal parts on which the panels weighing up to 1.5 tonnes rest.
Emergency workers were still at the nearly five-kilometre (three-mile) tunnel on Tuesday, but more than 24 hours after the cave-in, efforts had shifted from rescue to recovery.
Three vehicles were buried on Sunday when concrete ceiling panels crashed down inside the tunnel. Witnesses spoke of terrifying scenes as at least one vehicle burst into flames.
Emergency workers had collected five charred bodies -- three men and two women -- from a vehicle by early Monday. One report said the victims were all in their 20s.
They also recovered the body of a truck driver, identified as 50-year-old Tatsuya Nakagawa who reportedly telephoned a colleague immediately after the incident to ask for help.
Three other deaths have been confirmed, an elderly man and two elderly women, who were all in the same passenger vehicle, officials said.
"I offer my deepest condolences" to those affected, NEXCO Central president Takekazu Kaneko said. "First and foremost, the rescue operation is our priority. We are also inspecting our tunnels that use the same design."
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.
Chief Cabinet Secretary Osamu Fujimura said the government had ordered immediate action to shore up the transport system.
"The prime minister ordered the transport ministry to do its utmost in the rescue operation, to find out the cause at an early stage, to take thoroughly preventive measures against similar accidents," he said.
"We will have to make significant investment in public transportation systems and will need to ensure its durability. We need to review infrastructure as it ages."
Japan had an infrastructure boom in the 1960s and 1970s as the economy went through a period of spectacular growth.
But experts warn that as they age, many of tunnels and bridges will need to be replaced -- not an easy task for a government that already owes over twice what the economy makes in a year.