Police were investigating Wednesday the cause behind a fire at an apartment in Gunpo, Gyeonggi Province, that left four people dead and seven injured a day earlier.

Forensic officials conducted an on-site inspection of the apartment unit on the eighth floor of a 12-story building. It was under renovation and workers were replacing window frames when the blaze began, the Gunpo Police Station said.

Authorities suspect flammable materials, such as urethane foam, may have caught fire and started the deadly inferno. Backing that theory, local reports said neighbors heard multiple explosions before the house erupted in flames.

Of the four people who died, two were renovation workers and two were residents of other units in the building. The two workers died falling to the ground, and the two residents were found dead in staircases, apparently attempting to escape.

Of the seven injured, one is reportedly in critical condition.

In what appears to have been a similar case on April 29, a fire engulfed a warehouse under construction in Icheon, Gyeonggi Province, killing 38 workers and injuring 10 others. Officials then also identified unexpected explosions from urethane used in a basement as the cause of the fire.

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