Police have launched an investigation into a massive fire on a expressway that killed at least five people and injured some 30 others on Thursday.

A probe team at Gyeonggi Nambu Provincial Police, along with the National Forensic Service and fire authorities, conducted a joint forensic inspection of the accident site for about five hours from 11 a.m. Friday.

According to the probe team, the fire reportedly started from the lower part of a cargo compartment of a five-ton garbage truck and quickly spread to a noise-barrier tunnel along an expressway in Gwacheon, just south of Seoul.

Investigators collected battery, electrical wiring, and other debris from the waste disposal truck in order to determine the exact cause of the initial spark. They also inspected four other sedans in which the five people were found dead. 

Meanwhile, police booked the driver of the garbage truck on charges of involuntary manslaughter.