Investigators exit a hotel in Bucheon, Gyeonggi Province, Friday, after conducting a joint inspection of a blaze that occurred Thursday evening claiming seven lives. Yonhap

The rapid spread of toxic gas was largely responsible for the unexpectedly high number of casualties in a hotel fire in Bucheon, Gyeonggi Province, late Thursday, according to safety experts, Friday.

The incident claimed seven lives and left a dozen others injured, with three sustaining serious but non-life-threatening injuries, according to fire authorities.

The blaze broke out at 7:39 p.m. in a room on the eighth floor of the nine-story hotel.

Approximately 160 firefighters, 90 police officers and 70 emergency vehicles including pump cars were dispatched to the scene. The fire was fully extinguished as of 10:26 p.m., more than two hours after it began.

While the fire itself did not spread throughout the entire building, the higher-than-expected death toll was attributed to the rapid spread of toxic smoke, which filled the hotel, making evacuation difficult. There were 27 guests in the hotel at the time, many of whom struggled to escape as thick black smoke quickly enveloped the building.

The gas also caused widespread suffocation, leading to an increase in fatalities.

Most of the victims were found in the stairways and corridors on the eighth and ninth floors, near the source of the fire.

Given that many victims were found in stairways, there is a possibility that the fire door separating the stairways from the blaze was left open.

"People need to use stairways to evacuate when there's a fire, but if fire doors are open, toxic gases can fill the stairs, blocking the escape route," said Kong Ha-sung, a professor at the Department of Fire and Disaster Prevention at Woosuk University.

He warned that toxic gases can spread rapidly through stairways and emphasized that combustible materials in the hotel, such as mattresses, blankets and wooden furniture, likely accelerated the fire and increased the production of harmful fumes.

The hotel, built in 2003, did not have sprinklers installed, as they were not mandatory at the time.

Since 2017, fire regulations mandate that all new buildings with six or more floors must have sprinklers on every floor. However, this requirement does not apply retroactively, except for medical facilities and institutions for the elderly and children, such as nursing homes and daycare centers.

This gap in regulations leaves older buildings, like the hotel in Bucheon, particularly vulnerable to fire.

"Hotels and other older accommodations should also be required to install sprinklers retroactively," Kong emphasized.

Additionally, two guests — a man and a woman — died after jumping from the hotel onto an air mat below, which flipped over upon impact.

Fire authorities believe the guests jumped because smoke had filled the hotel, making it impossible to evacuate via the stairs or elevators.

The air mat had been set up correctly, but it overturned when one of the guests landed on its corner.

Kong noted that there is currently no specific protocol for securing air mats during rescues, stressing that such guidelines should be added to the safety manual.

Interior Minister Lee Sang-min, center, receives a briefing on the fire incident at the scene in Bucheon, Gyeonggi Province, Friday. Yonhap

Meanwhile, Interior Minister Lee Sang-min visited the scene of the fire on Friday morning and ordered an all-out effort to manage the aftermath of the incident.

"I extend my deepest condolences," Lee said. "The government will thoroughly investigate the cause of this tragedy and implement measures to prevent such accidents from happening again."

Cho Seon-ho, chief of the Gyeonggi-do Fire Services, said during an on-site briefing that a short circuit or other electrical issues are likely to have caused the fire.

Fire authorities revealed that a guest reported smelling something burning after entering Room 810, where the fire originated, and subsequently requested a room change. At the time of the fire, Room 810 was unoccupied.

Given the reported burning smell, fire officials suspect that the blaze may have been triggered by electrical factors, such as a short circuit, in the empty room.

Police conducted a joint inspection of the fire site on Friday starting at 11 a.m., with 31 officials from fire authorities and the National Forensic Service participating to determine the exact cause of the blaze.