The blaze on the 65th floor of Marina Bay Suites on Jan 13, which killed two security guards, was an accidental one and no foul play is suspected, a coroner's inquiry heard on Friday.
It had most likely started when cigarette embers ignited a trashbag of sawdust at the fireman's lift lobby there, said a police investigation report submitted to the court.
The fire then sustained itself on other materials and equipment nearby, including boxes and tins of paint. The lobby was next to one of the condominium's penthouse units, which was then undergoing renovation by workers from four different firms. Only 26 of the condominium's 221 units were occupied at the time.
Mr Sim Lai Huat, 45, and Ms Sooria Kala Kanaseon, 34, from Pico Guards, were burned to death when they went to investigate the fire.
Closed-circuit camera footage showed that the pair had taken the fireman's lift to the 65th floor just after 10pm that night.
Upon seeing the blaze, Mr Sim pressed the "close" button but the doors jammed, exposing them to flames and smoke.
The court heard that as smoke poured in, they tried to shelter themselves from the heat with the construction padding in the lift, but were overwhelmed.
The sawdust that had probably acted as fuel for the fire came from polishing works for the unit's parquet flooring. Referring to a bag of the material inside the unit that contained cigarette butts and a cigarette box, a Singapore Civil Defence Force report said the contents of the one in the lobby were probably similar.
Camera footage showed that two employees of lift company Schindler Singapore had been the last persons seen on the 65th floor before the fire, the police report said. While trying to repair a faulty passenger lift earlier that evening, they had taken the fireman's lift in question to the 65th floor and climbed a flight of stairs to a motor room one floor up.
Both denied to investigators that they had smoked in the area.
Mr Sim, who leaves a wife and three children, had been in the security industry for about 25 years and joined Pico last October. Ms Sooria Kala, a Malaysian, had been with the firm since last April.
State Coroner Marvin Bay adjourned the inquiry so more investigations can be conducted.
A Manpower Ministry report on the incident is still pending.
The hearing resumes on July 16.