The deadly blaze in a Mong Kok building, which claimed the lives of three people could have been caused by light fixtures in a flat, firefighters said on Friday.

The fire at Kin Wong Mansion on Sai Yee Street claimed the lives of a six-year-old boy and two men aged 53 and 80.

Director of Fire Services Andy Yeung said officers are still looking into what started the fire, dismissing rumours that an air conditioner unit was to blame.

"The department’s fire investigation team, together with the Electrical and Mechanical Services Department and the Government Laboratory, conducted tests at the scene Friday morning," he told reporters.

"Their investigation found that a row of light tubes beside the air conditioner was severely burnt, which could possibly have caused the fire. The relevant electronic items have been kept for further assessment to determine whether they are linked to the fire’s cause."

The department last inspected the building in June and found no fire safety hazards, Yeung added.

Some residents had said that the fire alarm failed to go off during the blaze.

The fire services chief said preliminary checks show that the building's fire alarm system was working, adding that it is possible no one activated the alarm when the blaze broke out.