Hong Kong ( CNN ) -- The mangled wrecks of cars being carried out of Japan 's Sasago Tunnel suggest there was little motorists could do to escape the sudden collapse of the ceiling above them . A day after the disaster , one main theory has emerged as to what caused the collapse , which killed nine people who were trapped in their cars by rubble or the flames that broke out shortly after . At a press briefing on Monday , the executive officer of the tunnel 's operator said it appeared that some `` anchor bolts '' used to secure concrete slabs to the tunnel ceiling were missing . `` There were parts of concrete ( slabs ) where bolts had fallen off , '' Ryoichi Yoshizawa said , according to a spokesman for Central Japan Expressway Company or NEXCO-Central . `` The aging of the bolts or the concrete slabs could be a potential cause ( of the collapse ) , '' Yoshizawa said . He did not say how many bolts were found to be missing or how they came to be loose . Yoshizawa added that while regular checks had been performed on the tunnel , they were visual checks and there was no physical testing . Read more : Nine bodies recovered from tunnel . Emergency inspections have been ordered on 49 tunnels across the country with the same ceiling structure , according to the Ministry of Land , Infrastructure , Transport and Tourism . There are 1,575 highways tunnels in Japan and around a quarter of those are more than 30 years old , including the Sasago Tunnel which opened in 1977 , the ministry said . The tunnel 's ceiling gave way on Sunday morning at around 8 a.m. local time . Witnesses recalled the horror of smoke filling up the tunnel as huge concrete slabs rained down on traffic below . Japanese highway police said Monday the section of concrete that fell was 110 meters ( 360 feet ) long . Charred bodies were pulled from the debris , including five from a single station wagon . Three others were in a burned vehicle , according to a police spokesperson , while another body was found in a truck . `` It was terrifying . I do n't think I could ever drive through the tunnel again , '' one shaken survivor told TV Asahi , as black and white video released by NEXCO showed rescue workers in flashlight-topped helmets stepping over rubble . The tunnel has been closed for the removal of debris and while experts ascertain the risk of a secondary accident . NEXCO says it 's unsure how long the process could take . Speaking to reporters at the scene , Motohiro Takamisawa , the chief of NEXCO 's Otsuki Safety Center , also referred to a potential problem with the bolts securing the tunnel 's ceiling slabs . `` At this moment we 're presuming that the top anchor bolts have come loose , '' he was reported as saying . Takamizawa added that the bolts had n't been changed since the tunnel first opened in the late 1970s . However , a company spokesman told CNN that Takamizawa 's comments should not be interpreted as the company 's official statement and that it could not confirm whether that was the case . One expert told Asahi TV said that it 's possible that years of traffic vibrations had contributed to the tunnel 's collapse . `` Over the course of 35 years , all the shaking caused by cars has probably caused the bolts and nuts in the tunnel to loosen . As a result they fell off , '' said Hiroshi Chikahisa , head of the Geosystem Engineering Institute at Yamaguchi University . Immediately after the disaster , a company spokesman said the Sasago Tunnel , located about 80 kilometers ( 50 miles ) west of Tokyo , was subject to annual inspections with more detailed checks every five years . It had been checked in the last couple of months . At the company 's Monday briefing , a NEXCO spokesman said , `` There was no record that we have conducted the tapping inspection at top of the ceiling in the tunnel . '' He was referring to a method used to identify potential damage within concrete structures that was mentioned in the company 's 2011 annual report . It says , `` although hammer tapping test is commonly carried out to investigate concrete structures , it takes enormous time and cost to conduct the test on all concrete structures we have . '' Instead , it says the company inspects concrete structures using infrared cameras , an inspection technology which measures the difference in temperature between `` sound conditions and damaged areas '' to detect potential weak points . There as no explicit reference to testing carried out on tunnels . The Sasago Tunnel runs for 4.7 kilometers along a stretch of the Chuo Expressway which runs for 367 kilometers , through a mountainous region , connecting Tokyo with the Nagoya in the Chubu region of Japan . Its operator , NEXCO-Central is one of three companies started in 1995 after the privatization of Japan 's Highway Public Corporation . NEXCO-Central manages more than 1,700 kilometers of expressways in Tokyo and the Chubu , Hokuriku and Kinki regions , used by almost 1.9 million cars on any given day . Journalist Toshi Maeda , CNN 's Junko Ogura and Alex Zolbert contributed to this report .
Collapse of Sasago Tunnel is being blamed on failure of `` anchor bolts '' The bolts secure concrete slabs to the ceiling of the tunnel . Nine people died when concrete slabs fell on traffic on Sunday morning . A total collapse of a bridge in central Japan has killed at least nine people, including two children, and injured more than 100 others.