Firefighters examine cars destroyed by a fire the previous day inside an expressway noise-barrier tunnel, just south of Seoul, Dec. 30. Yonhap

Police on Friday launched a full-scale investigation into the previous day's deadly expressway tunnel fire in Gwacheon, just south of Seoul, which killed five people and injured 37 others.

The Gyeonggi Nambu Provincial Police Agency said it began an on-spot inspection of the accident scene, about 15 km south of Seoul, in the morning, together with the National Forensic Service and fire authorities.

The blaze occurred early Thursday afternoon inside a noise-barrier tunnel in the Gwacheon section of the Second Gyeongin Expressway that connects the western port city of Incheon to Seongnam.

The fire reportedly started from a 5-ton garbage truck and rapidly spread to engulf soundproof awnings, leaving several dozens of cars trapped inside the tunnel.

The police investigation is expected to focus on the cause of the truck fire and the cause of the deaths of the victims in five vehicles in the opposite lane.

Firefighters examine cars destroyed by a fire the previous day inside an expressway noise-barrier tunnel, just south of Seoul, Dec. 30. Yonhap

The accident scene, which includes 45 vehicles destroyed by the fire, has been preserved intact after the blaze was extinguished late Thursday afternoon.

In addition, contractors involved in the construction of the noise-barrier tunnel and a corporation responsible for the operation of the expressway are expected to come under investigation to check whether there were any problems in the overall process of its construction and maintenance.

Police will also look into why the tunnel entry blocking device did not work at the time of the accident.

Meanwhile, the Gyeonggi police agency booked the truck driver on charges of involuntary manslaughter earlier in the day after questioning him the previous day.

The driver reportedly told police that his truck suddenly burst into flames when he was driving inside the tunnel. (Yonhap)