TOKYO: Japan ordered inspections of ageing highway tunnels on Monday after a fiery collapse that killed nine people, as suspicion over the cause of the accident centered on decaying ceiling supports.

The government pledged a thorough review and said that “significant investment” would likely be required in the motor-way 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 Mt. Fuji.

Footage from inside the tunnel showed that the 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.

Engineers on Monday began inspections at three other tunnels in the region with the same design, as well as at Sasago.

Also, the Ministry of Land, Infrastructure and Transport ordered emergency inspections of all 49 highway tunnels nationwide that have the same design.

Nexco said that 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 tons rest.

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 on Monday. One report said that 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.

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 that the government had ordered immediate action to shore up the transport system.

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 is worth in a year.