Firefighters had mostly doused the blaze that had torn through the Wang Fuk Court housing complex in the northern district of Tai Po, which had been undergoing renovations.
Rescuers had been battling intense heat and thick smoke for more than a day as they fought to reach residents still feared trapped on the upper floors.
Video showed firefighters with flashlights searching through the charred remains of the towers on Thursday night.
A distraught woman carrying her daughter's graduation photograph searched for her child outside a shelter, one of eight that authorities said are housing 900 residents.
"She and her father are still not out yet," said the 52-year-old, who gave only her surname, Ng, as she sobbed.
"They didn't have water to save our building."
Police arrested two directors and an engineering consultant of Prestige Construction, a firm contracted to perform maintenance on the buildings.
Police said those arrested were under suspicion of manslaughter for using unsafe materials.
"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," police superintendent Eileen Chung said.
Prestige did not answer repeated calls for comment.
Police seized bidding documents, a list of employees, 14 computers and three mobile phones in a raid of the company's office, the government added.
The confirmed death toll rose to 75 as of late on Thursday, the South China Morning Post reported, citing the fire department.
That made it Hong Kong's deadliest fire in 77 years.
Hong Kong's leader, John Lee, said the government would set up a HK$300 million ($A60 million) fund to help residents.
Chinese companies and groups, including car makers Xiaomi, Xpeng and Geely as well as the charity foundation of Alibaba's founder Jack Ma and Tencent announced donations.
The eight blocks of the tightly packed complex of 2000 apartments were home to more than 4600 people in the financial hub, which is struggling to overcome chronic shortages of affordable housing.
The apartments were under renovation and clad in bamboo scaffolding and green mesh.
Police also said they found foam material sealing some windows on one unaffected building, installed in year-long maintenance work.
The city's development bureau has discussed gradually replacing bamboo scaffolding with metal scaffolding as a safety measure.
Authorities said they had doused the flames in four of seven affected blocks, with those in the rest brought under control.
"We bought in this building more than 20 years ago," said a 51-year-old resident surnamed Wan.
"All of our belongings were in this building, and now that it has all burned like this, what's left?"
An online app showed reports of missing persons submitted through a linked Google document that detailed residents of individual towers and rooms.
It includes descriptions such as "Mother-in-law in her 70s, missing" or "one boy and one girl" or "Rooftop: 33-year-old male."
One simply reads "27th floor, room 1: He is dead."
Reuters could not independently verify the information on the app.
Hong Kong, one of the world's most densely populated cities, is scattered with high-rise housing complexes.
Tai Po, near the border with mainland China, is an established suburban district and home to about 300,000.
Occupied since 1983, the complex is under a government-subsidised home ownership scheme, according to property agency websites, a lifeline for the city's middle-income families.
