A fire at a warehouse in Icheon, Gyeonggi Province, broke out and turned deadly because safety regulations were disregarded, police said Monday.

The Gyeonggi Nambu Provincial Police Agency said in a press briefing Monday that the fatal accident in late April occurred because welding activities at a facility under construction were carried out in an unsafe manner.

After a joint investigation with seven government agencies that kicked off 48 days after the deadly fire, police concluded that the fire started in a sub-basement during construction work involving urethane, a combustible chemical used for insulation.

Sparks from the welding splashed onto the urethane that covered the ceiling and walls. The fire got bigger because it spread upstairs, where there were higher concentrations of oxygen.

Police said more people died because a large number of workers had been dispatched to the construction site to complete it faster, and because hallways were blocked to keep out the cold weather. On the day of the fire, 67 workers were at the site, around double the number of workers dispatched there on a normal day.

The fire tore through a warehouse that was under construction for HanExpress, killing 38 and injuring 10 others. Among those killed were one Chinese worker and two Kazakh workers.

Under South Korean law, welders are required to work in pairs and to make sure nearby areas are covered in fire-resistant materials. Police said these rules were not followed and that the supervisor -- who was not present when the fire started -- had failed to instruct the workers on safety procedures.

The police referred the case to the prosecution, recommending involuntary manslaughter charges against five HanExpress employees as well as 19 employees of the construction company and its partners. Police requested arrest warrants for nine of the accused people.

Police have said they will continue the investigation to determine whether any additional regulations were violated.

By Ko Jun-tae (ko.juntae@heraldcorp.com)