The ministry's Institute of Forensic Sciences found that a problem in the electric circuit of an LED light bulb on the second floor, where several products and materials are stored, caused the fire.

The five-hour blaze that started at 6 p.m. on August 28 destroyed a third of the inventory at the 6,000-square-meter warehouse belonging to the Rang Dong Light Source and Vacuum Flask JSC in Thanh Xuan District.

About 15.2-27.2 kilograms of mercury were released into the environment following the fire, the Vietnam Environment Administration said.

Contamination work at the fire site started last Thursday and about 65 tons of waste were removed from the wreckage as of Monday. The cleanup is expected to take another week.

People living nearby began moving out of their homes due to health concerns but authorities have tried to allay their fears.

Hanoi's health officials said no mercury poisoning case has been detected among 1,000 health checks done in the vicinity of the fire.

Minister of Natural Resources and Environment Tran Hong Ha last Thursday stated that areas surrounding the blaze-gutted Rang Dong warehouse are safe now.

Rang Dong, a leading manufacturer of lighting products and equipment, has estimated the loss from the fire at VND150 billion ($6.4 million).