Police said Saturday they will seek the arrests of several subway officials for negligence in a devastating subway train fire that killed at least 133 people in South Korea's third-largest city.

The widening investigation comes as victims' relatives visited the burned-out station and fingered through the wreckage for personal items of loved ones.

It was the first time victims' relatives were allowed onto the charred platforms since Tuesday's disaster, which sparked nationwide concern about lax safety standards.

Police said Saturday they will seek a homicide warrant for the mentally disturbed man who allegedly ignited the fire in attempt to commit suicide. If convicted, he faces a minimum of seven years behind bars. The maximum sentence is death.

A Daegu police investigator who identified himself only as Sgt. Yoon said police will also pursue the arrests of six subway officials, including the drivers of the two trains that caught fire. Police intend to file warrants on charges of homicide by negligence, Yoon said.

Those charges carry a penalty of up to five years in prison.

The driver of the train that first caught fire is being eyed for failure to report the fire to subway authorities quickly enough, Yoon said.

Police said Friday that the driver of the second burning train apparently fled without opening the doors for his passengers, leaving dozens to die.

Most of the dead were killed in that train, which pulled into the station after a first train had already caught fire. The conductor told police he thought that the doors were open as fire engulfed the cars and that passengers had evacuated, so he took the key used to control the doors and fled to safety.

Police said that of the six cars in the train, the doors were open on only two of them, meaning that dozens of passengers were left behind, trapped in raging flames and toxic smoke.

Had the train been equipped with fire-resistant seats and floor tiles, critics said, it probably would not have burst into flames. An apparent lack of adequate emergency lighting left victims groping in the dark after the lights went out.

In South Korea, police must apply to the courts for permission to issue arrest warrants. The process can take several days.

About 150 people, including victims' relatives, took part in a half-hour march through Daegu to commemorate the victims and call for punishment of officials on Saturday. They blamed city officials and subway managers for apparently lax safety standards that may have contributed to the high death toll.

Holding placards that called for the arrest of Daegu's mayor and the head of the subway corporation, grieving relatives then descended into the blackened subway station. Many burst into tears as they entered, yelling out the names of the dead.

Inside the station, some climbed onto the tracks and sifted through the ashen debris for signs of lost loved ones.

The group also called for the city to suspend operation of the subway until safety concerns had been addressed.

Police watched over the demonstration, but there were no clashes.