A devastating fire at a residential complex in Hong Kong has resulted in 55 fatalities, marking one of the city’s deadliest incidents in recent history. Firefighters are still engaged in efforts to extinguish the blaze.

On Thursday, Hong Kong officials confirmed that 51 individuals were found deceased at the scene, with the total death toll now including four more who succumbed to their injuries in the hospital.

The Wang Fuk Court complex in Tai Po district, located near the mainland border, continues to emit dense smoke as the fire rages on.

Fire crews have been working tirelessly since the fire erupted on Wednesday afternoon. The flames quickly engulfed seven of the eight buildings within the complex.

Authorities announced that fires in four of the structures have been successfully extinguished, while efforts to control the remaining three buildings are ongoing. They anticipate that operations may continue into the evening.

One firefighter was among the people confirmed dead, officials said.

More than 70 people were injured, according to the city’s Hospital Authority, many suffering from burn and inhalation injuries.

Resident Lawrence Lee was waiting for news about his wife, who was still trapped in their apartment.

“When the fire started, I told her on the phone to escape. But once she left the flat, the corridor and stairs were all filled with smoke and it was all dark, so she had no choice but to go back to the flat,” he said, as he waited in one of the shelters overnight.

Winter and Sandy Chung, who lived in one of the towers, said they saw sparks fly around as they evacuated Wednesday afternoon. Although they were safe, they were worried about their home. “I couldn’t sleep the entire night,” Winter Chung, 75, told The Associated Press on Thursday.

Three men, the directors and an engineering consultant of a construction company, were arrested on suspicion of manslaughter.

Police have not directly named the company where they work.

“We have reason to believe that those in charge of the construction company were grossly negligent,” said Eileen Chung, a senior superintendent of police.

Police on Thursday also searched the office of Prestige Construction & Engineering Company, which the AP confirmed was in charge of renovations in the tower complex. Police seized boxes of documents as evidence, according to local media. Phones for Prestige rang unanswered.

Authorities suspected some materials on the exterior walls of the high-rise buildings did not meet fire resistance standards, allowing the unusually fast spread of the fire.

Police also said they found Styrofoam — which is highly flammable — attached to the windows on each floor near the elevator lobby of the one unaffected tower.

It was believed to have been installed by the construction company but the purpose was not clear.

Secretary for Security Chris Tang said they would investigate the materials further.

The fire started on the external scaffolding of a 32-story tower, then spread on the bamboo scaffolding and construction netting to the inside of the building and then to the other buildings, likely aided by windy conditions.

Firefighters aimed water at the intense flames from high on ladder trucks, but conditions for fighting the fire and rescuing people remained challenging.

A fire safety expert said the incident “is quite shocking,” as regulations in general require buildings to be spaced apart to keep fires from spreading from one building to the next. “Typically, they don’t spread beyond the building of origin,” said Alex Webb, a fire safety engineer at CSIRO Infrastructure Technologies in Australia, saying the materials police cited could explain why the fires spread.

Bamboo scaffolding is a common sight in Hong Kong at building construction and renovation projects, though the government said earlier this year that it would start phasing it out for public projects because of safety concerns.

The housing complex consisted of eight buildings with almost 2,000 apartments for about 4,800 residents, including many older people.

It was built in the 1980s and had been undergoing a major renovation.

About 900 people were evacuated to temporary shelters overnight, and Hong Kong leader John Lee said around midnight that contact had been lost with 279 people.

Rescues were continuing, but an updated figure wasn’t available by midday Thursday.

Lee said separately on Thursday that officials will be carrying out immediate inspections of all housing estates across the city that are undergoing major renovation work to ensure scaffolding and construction materials meet safety standards.

Chinese leader Xi Jinping expressed condolences to the firefighter who died and extended sympathies to the families of the victims, according to state broadcaster CCTV.

He also urged efforts to minimize casualties and losses.

The fire was the deadliest in Hong Kong in decades. In November 1996, 41 people died in a commercial building in Kowloon in a fire that lasted for around 20 hours.