Investigators are carrying out an additional on-site probe at an oil storage facility where a tank caught fire near Seoul on Sunday. 

A police official said that Thursday's on-site probe will focus on the oil storage facility, including the density of the oil mist formed inside the tank.

The police arrested an immigrant worker from Sri Lanka who is believed to have accidentally caused the fire after releasing a sky lantern he found, which subsequently landed in the oil facility. 

The prosecution has twice refused to issue an arrest warrant, citing lack of evidence. 

Police did, however, ban the Sri Lankan national from traveling and will question him further without detention.

[Photo : YONHAP News]