Victims' relatives are demanding answers

They say prosecutors will probably issue arrest warrants next week following Tuesday's fire at the station in the city of Daegu.

The suspected arsonist, a man with a history of mental illness, has already been detained.

The widening of the investigation came as grieving relatives of the dead and missing were allowed into the station area for the first time, during a demonstration calling for the underground service to be suspended until all safety issues are resolved.

About 150 people took part in the demonstration, accusing city officials and railway managers of allowing poor safety standards.

Some protesters held placards calling for the arrest of the city mayor and the head of the subway corporation.

Passengers trapped

Many of the victims' relatives wept as they entered the burnt-out station, searching through the ashes for any personal items that might have belonged to their loved-ones.

Human errors could have increased the death toll

Police on Friday said the driver of a second train, which pulled into the station after the fire started, was suspected of fleeing without opening the door for passengers.

Questions have been asked about why the second train - where most of the victims died - was allowed into the station in the first place.

Witnesses said the fire started when a male passenger lit a container of flammable liquid as his train pulled into a station.

Police say the suspect, 56-year-old Kim Dae-hwan, is too ill to be interrogated.

Mr Kim has reportedly said that he wanted to commit suicide by self-immolation, but did not want to die alone.

If convicted, he faces a minimum prison sentence of seven years. The maximum sentence is death.