The testimony came at the start of the fifth round of evidentiary hearings conducted by the independent committee investigating the devastating fire in Tai Po.
In the afternoon session, Lee Wing-man, from the forensic science division of the Government Laboratory, presented her findings on the origin and cause of the blaze.
According to Lee, the fire started on a platform located outside flats 104 and 105.
Both units adjacent to the seat of the fire sustained severe damage, with door frames completely burned away.
She ruled out the possibility that the fire had spread from inside the units, and noted that the platform where the fire originated was not generally accessible to residents.
In her report, Lee stated that the fire was most likely accidental and originated from a source that was glowing – such as a lit cigarette end – that ignited combustible materials like cardboard boxes at the scene, though direct evidence of the cause could not be identified.
“It was the only suspicious cause of fire that we could find,” she said, adding that this conclusion was reached through a process of elimination.
She said that numerous cigarette butts and cardboard boxes were found on the platform, materials capable of producing smouldering – a slow, low-temperature, flameless form of combustion.
Lee also told the hearing that laboratory tests had ruled out arson.
“Nothing of significance was found with respect to recovery of ignitable liquid residues from all debris samples seized from the seat of fire,” she indicated in her report.
She added that the possibility of an electrical fault was also dismissed.
Lee also said that the Government Laboratory conducted several tests on scaffolding mesh and plastic sheet samples, all of which failed to meet national fire-retardancy standards.
When exposed to flame, she said, both the canvas and safety nets caught fire.
“Burning droplets generated from the combustion of safety nets would serve as new ignition source sparks new fire,” Lee pointed out.
She also said tests on foam and plastic boards showed “no signs of self-extinguishing”.