Chief Secretary Eric Chan on Monday said tests had confirmed that seven of the mesh netting samples taken from Wang Fuk Court were not flame-retardant – and that contractors are believed to have deliberately hidden their use of substandard material from inspectors.

Chan, who chairs the working group investigating the deadly fire in Tai Po, said investigators had collected 20 samples from four blocks of the estate in Tai Po over the past couple of days.

He said the non-compliant netting was deliberately placed in locations that only firefighters could reach.

"Officers found that a total of seven samples taken from Wang Tai House, Wang Tao House, Wang Yan House and Wang Chi House didn't reach fire-retardant standards. The samples were taken from different spots across the higher, lower and middle floors of the blocks," Chan said.

Thirteen people, aged 44 to 77, have been arrested on suspicion of manslaughter.

ICAC Commissioner Danny Woo said 12 of the 13 had also been arrested on suspicion of corruption.

He said some suspects had originally bought a large quantity of substandard netting and later purchased a smaller lot of higher quality mesh.

"In late October, there was a fire in Central involving the mesh netting draped over bamboo scaffolding. The suspects were worried their own netting would be subject to spot checks," Woo said.

"For the sake of satisfying test requirements, they bought 115 rolls of mesh netting at the price of HK$100 per roll, with the rolls covering an area of 3,700 square metres.

"In what we believed is an attempt to fool inspectors, the compliant netting was installed at the base of the buildings' scaffolding, and the netting passed tests."

Secretary for Security Chris Tang said samples taken previously and passed fire resistance test were from the only block that didn't catch fire – Wang Chi House.

He said preliminary investigations found that the mesh netting and styrofoam boards used to seal windows had caused the rapid spread of the blaze.

Tang also noted that there had been certain individuals attempting over the past few days to endanger national security at such a painful time for the city by using fake information to incite dissatisfaction towards the government.

The information, he said, included claims that the rescue methods used by firefighters were inappropriate, or that first responders had been unhappy.

Tang said details of the national security operations could not be disclosed but stressed that authorities would take all necessary actions over seditious comments online.

Officials also said the Housing Department had taken netting samples from 300 blocks throughout the city that were currently undergoing external repairs.

On-site inspections of all such buildings are due to be completed by Sunday.