A huge fire still burning in a Hong Kong apartment complex that has killed at least 75 people and left almost 300 missing may have been spread by unsafe scaffolding and foam materials used during maintenance work, police said on Thursday.

Working through the night, firefighters were struggling to reach residents potentially trapped on the upper floors of the Wang Fuk Court housing complex due to intense heat and thick smoke from the fire that erupted on Wednesday afternoon.

The tightly packed complex in the northern Tai Po district has 2,000 apartments in eight blocks, housing more than 4,600 people.

By Thursday morning, authorities said they had brought the fire in four blocks under control, with operations continuing in three blocks.

Video from the scene showed flames still leaping from at least two of the 32-storey towers sheathed in bamboo scaffolding and green construction mesh, as heavy smoke billowed into the sky.

Police said in addition to the buildings being covered with protective mesh sheets and plastic that may not meet fire standards, they discovered some windows on one unaffected building were sealed with a foam material, installed by a construction company carrying out maintenance work.

“We have reason to believe that the company’s responsible parties were grossly negligent, which led to this accident and caused the fire to spread uncontrollably, resulting in major casualties,” Eileen Chung, a Hong Kong police superintendent, said.

Three men from the construction company, two directors and one engineering consultant, had been arrested on suspicion of manslaughter over the fire, she added.

The green construction mesh and bamboo scaffolding used on the buildings are a mainstay of traditional Chinese architecture, but have been subject to a phase-out in Hong Kong since March for safety reasons.

A firefighter was among the 75 killed, with 45 people in hospital in critical condition, Hong Kong police told a press conference before dawn on Thursday.

The death toll is now the highest in a Hong Kong fire since 1948, surpassing the 41 killed in a blaze in a commercial building in the Kowloon district in November 1996.

The latest fire has prompted comparisons to the Grenfell Tower inferno that killed 72 people in London in 2017. That fire was blamed on firms fitting the exterior with flammable cladding, as well as failings by the government and the construction industry.

“The priority is to extinguish the fire and rescue the residents who are trapped,” Hong Kong leader John Lee told reporters earlier. “The second is to support the injured. The third is to support and recover. Then, we’ll launch a thorough investigation.”

Some 279 people were uncontactable, and 900 were in eight shelters, he added.

One 71-year-old resident surnamed Wong broke down in tears, saying his wife was trapped inside.

Harry Cheung, 66, who has lived at Block Two in one of the complexes for more than 40 years, said he heard a loud noise about 2:45pm and saw fire erupt in a nearby block.

“I immediately went back to pack up my things,” he said.

“I don’t even know how I feel right now. I’m just thinking about where I’m going to sleep tonight.”

Another long-time resident, a woman surnamed Chu, said she still had not been able to contact her friends who live on the next block. After staying over at a friend’s place on Wednesday night, the 70-year-old came back to see her home still burning.

“We don’t know what to do,” she said.

President Asif Ali Zardari, on Wednesday, expressed his deep sorrow over the loss of lives in a residential fire in Hong Kong that claimed several lives.

“The president extended heartfelt condolences to the bereaved families and conveyed sympathy to all those affected by the tragedy.

In a message of condolences to President Xi Jinping and the people of China, he reaffirmed Pakistan’s solidarity at this difficult time,” President Secretariat Press Wing said in a press release.