Concerns over the safety of soundproof tunnels are mounting after a fire in a highway tunnel in Gwacheon, Gyeonggi Province, on Thursday left five dead and 41 injured.

The blaze that engulfed and gutted the bulk of the 830-meter-long tunnel-shaped structure is reported to have started with a cargo truck, where a fire broke out for unknown reasons. Dozens of cars were trapped under the structure along the road as the fire spread to the soundproof walls and lasted two hours before firefighters put out the flames.

On Sunday, police raided the waste collection company to which the truck belongs in order to identify the cause of the fire.

According to police, the investigation is focused on whether the fire was caused by outdated parts or a lack of maintenance for the truck, while other possible factors are also to be explored. Police retrieved the remains of wiring linked to the truck's battery on Friday for further analysis.

Police booked the truck driver on suspicion of occupational negligence and said additional raids might be conducted at related sites if necessary.

The probe came after President Yoon Suk-yeol ordered related government agencies to conduct an all-out inspection of sound barriers and other road structures to see whether the component materials are sufficiently fire-safe.

Inspections and investigations should be duly conducted to prevent future highway fires that could result in massive casualties, but what is regrettable is that there had been earlier warnings about the soundproof tunnel, only for them to be ignored in a situation reminiscent of other high-profile accidents.

Experts point out that safety standards in the tunnel were not strict enough and that the fire safety barrier did not work, resulting in greater damage.

Soundproof tunnels are fundamentally vulnerable to fire accidents as they are structured in a way that traps heat and hazardous gases when a blaze breaks out inside.

While the structures need to be strictly managed, current fire prevention regulations reportedly have a host of problems, including the absence of fireplugs.

According to the Ministry of Land, Infrastructure and Transport, soundproof tunnels are not categorized as conventional tunnels subject to legal regulations designed to prevent fire. As a result, soundproof tunnels -- chiefly designed to resolve noise problems for apartment complexes nearby -- do not need to install firefighting equipment. Nor do they need to undergo strict inspections.

Conventional tunnels are required to use inflammable materials, and soundproof tunnels are supposed to follow the same rule, but detailed standards are missing, according to experts.

The Korea Expressway Corporation Research Institute has published two studies over the soundproof materials used on highways. One study, released in 2012, warned that the polymethyl methacrylate (PMMA) material used in the soundproof tunnel engulfed by the blaze Thursday was prone to catch fire more easily and spread faster than polycarbonate materials. The institute also issued a warning in its 2018 study, claiming that the PMMA often used for soundproof tunnels due to its lower price is more vulnerable to fire accidents due to its flash point of 280 degrees Celsius, compared with polycarbonate, the flash point of which is 450 degrees Celsius.

There were two fire accidents involving soundproof tunnels in 2020. Even though there were no casualties, authorities should have taken more proactive action to fix the flaws related to soundproof materials.

But it was not until July 2022 that the Land Ministry belatedly began a formal research project into fire-related regulations for soundproof tunnels in accordance with the recommendation of the Board of Audit and Inspection.

There are over 100 soundproof tunnels operated by the government and regional administrations. The Land Ministry said it plans to inspect all of the nation's soundproof tunnels. The government, often criticized for its belated moves to fix safety issues, must take a more proactive approach and lose no time strengthening safety standards to prevent similar fatal accidents.