Central Nippon Expressway Co in charge of a collapsed tunnel in Japan has admitted workers only carried out visual checks on metal bolts used to anchor concrete slabs in the tunnel roof.

Central Nippon Expressway Co faces a police investigation after at least nine people were killed in Sasago Tunnel, 50 miles west of Tokyo, when ceiling slabs fell on to moving vehicles inside the tunnel.

Transport minister Yuichiro Hata has instructed companies to check all 49 tunnels in Japan that have similar concrete slab structures.

Rescue efforts were suspended on Monday morning while work is carried out to support the remaining slabs and prevent further collapses.

An inspection of the tunnel’s roof in September found nothing amiss, according to company official Satoshi Noguchi.

But company officials told Japanese newspaper Asahi Shimbun workers only carried out visual checks on whether bolts were securely in place, rather than tapping parts with a hammer to detect signs of corrosion.

They also said the September inspection only involved the use of torches and binoculars to check the panels.

The tunnel opened in 1977 and the collapse has been blamed on the ageing of the metal bolts.

Ryoichi Yoshikawa, executive officer of Central Nippon Expressway, said: “Based on the fact that the accident occurred 35 years after the tunnel was completed, we believe ageing was the reason.”

Some of the nine-inch bolts were found near the site of the collapse, officials said.

Two vehicles caught fire inside the tunnel after the collapse on Sunday morning, with heavy smoke initially hampering rescue efforts.

The location of the collapse, around one mile into the three-mile tunnel, also made recovery difficult.

It is unclear if there are survivors inside.

An estimated 270 concrete slabs – each weighing 1.4 metric tons – collapsed over a stretch of around 110 metres.

Drivers described narrow escapes from falling debris, and a long walk through the darkness after abandoning their cars.

“When I was driving in the tunnel, concrete pieces fell down suddenly from the ceiling,” a man in his 30s told public broadcaster NHK.

“I saw a crushed car catching fire. I left my car and walked for about an hour to get out of the tunnel.”

Another said: “I could hear voices of people calling for help, but the fire was just too strong.”

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