The proximity of a mattress to the source of last week's deadly fire at a hotel in Bucheon, Gyeonggi Province, appears to have significantly contributed to the rapid spread of the blaze, authorities investigating the case said Monday.

The preliminary investigation suggests an electrical fault in the air conditioning unit as the likely cause, with sparks igniting the nearby sofa and the mattress. Fire authorities suspect that overheating or dust in the aging device precipitated the fault.

Camera footage obtained by the fire authorities showed the fire ignited in room 810 on the eighth floor at around 7:34 p.m. Thursday, after a guest heard crackling noises and smelled burning from the air conditioning unit.

The guest requested a room change and was moved to the sixth floor, but the door to room 810 was left open. This allowed thick smoke to quickly fill the eighth-floor hallway and spread to other floors, severely hindering evacuation efforts within just one minute and 23 seconds.

Research by the Korean Disaster Prevention Association indicates that fires involving bed mattresses grow much more quickly than usually. Specifically, mattresses can spread flames 490 times faster than televisions, 230 times quicker than wooden desks, and nine times faster than dressers. Once burning, mattresses also release substantial toxic gases, complicating evacuation.

Fire authorities confirmed that the hotel, built in 2003, did not have sprinklers installed in the rooms. The building was not subject to the 2018 regulation requiring sprinklers in newly constructed buildings over six floors. Unlike some medical facilities, older hotels are not required to retroactively install them. The absence of sprinklers in the hotel critically exacerbated the incident, fire authorities said.

Among the fatalities were two people who died after attempting to escape by jumping onto an inflatable air mattress positioned by firefighters, which overturned as they landed on it.

Autopsy reports from the National Forensic Service indicate that the other five victims died from carbon monoxide inhalation.

The Bucheon Hotel Fire Investigation Unit at the Gyeonggi Nambu Provincial Police Agency has detained two individuals: The actual operator of the hotel and the nominal owner. They have been charged with professional negligence resulting in death and injury.

The investigation is focusing on their failure to adhere to adequate fire safety standards, although investigators haven't publicly confirmed the details of this failure.

Article 268 of the Criminal Act, which addresses death and injury by occupational or gross negligence, is the main legal provision for manslaughter in a corporate setting. It is punishable by up to five years in prison or a fine of up to 20 million won ($14,800). Authorities have also imposed a travel ban on both individuals to ensure their availability for further interrogation.

The police and fire authorities are still investigating to confirm the cause of the fire -- which led to seven deaths and a dozen injuries -- based on detailed analysis results from the National Forensic Service. To date, authorities have interviewed 15 people, including surviving guests and hotel staff, to piece together the complete sequence of events leading to the disaster.