The blaze Monday gutted the 28-story building, leaving 70 injured and dozens unaccounted for, and prompted a belated crackdown on illegal construction work and lax fire precautions. It also is raising alarm over widespread use of flammable insulation used to retrofit buildings to meet new energy standards.

“The accident should not have happened and was completely avoidable,” Luo Lin, head of the State Administration of Work Safety, said in remarks posted Thursday on his agency’s website.

Luo listed a litany of problems with the government-sponsored energy-saving project: illegal use of unlicensed subcontractors, poor construction site management, lax local safety supervision and use of highly flammable nylon netting, insulation and other materials that caused the fire to spread out of control throughout almost the entire building.

Anguished family members and others angry over the government’s handling of the disaster are demanding answers.

“The government owes us a reasonable explanation and serious investigation. They have already lost the chance to apologize to those who were killed,” said Wang Lianguo, a neighbor who said he spotted the fire while doing laundry.

Authorities say they have identified only 26 of the 53 bodies taken from the building. More than 30 people reportedly remain missing from the blaze. Most of the dead perished in their homes, suffocated by toxic smoke, as firefighters struggled to break through metal security doors.

Officials have not said how many they believe are unaccounted for, although Shanghai’s fire chief said the building was thoroughly searched after the fire was extinguished.

The inferno was set off by sparks from welding intended to affix insulating materials to the outer walls of the building that hit the nylon netting hung as a safety precaution from bamboo scaffolding, Luo said. Police have detained eight suspects, including four they said were welders working without proper qualifications.

The disaster was a blow to Shanghai, China’s business capital and one of the country’s most modern, well-run cities, coming less than month after it claimed success in hosting a World Expo that drew a record 72 million visitors.

China has been tightening its energy standards, partly to meet its pledges on climate change and also to reduce waste and curb soaring consumption of costly and scarce energy resources.

Safety and other environmental issues apparently are being overlooked as local officials and businesses rush to jump on the energy-saving bandwagon with retrofitting projects that, inevitably, are fresh opportunities for the corruption and corner cutting endemic to construction work in this building-crazed country.

While the nylon netting — illegal but widely used on construction projects — appeared to be the main vehicle for the fire’s spread, the polyurethane foam insulation being layered on the apartment building’s outer walls was also highly flammable.

Monday’s fire, and a similar one triggered by fireworks that destroyed a luxury hotel at the Beijing headquarters for CCTV, China’s main television network, show the need for tighter controls on use of new energy efficient materials, said Li Hua, an engineer and researcher with the China Academy of Building Research.

“Building insulation materials are relatively new in China, but widely used. The government should supervise the quality of these materials, ensuring they pass fire prevention standards,” he said.

Use of polyurethane foam and other insulating materials is closely regulated in other countries to minimize well-known fire risks with flame retardants and fire barriers. Such materials are not allowed in skyscrapers.

“Buildings are vulnerable to fire during installation of these insulation materials. It’s safer when another layer of coating is added to prevent fire,” Li said.

The main government-owned companies assigned to retrofit the Shanghai apartment building farmed the work out to other, smaller contractors who employed nonprofessional laborers, further complicating matters. Local reports cite residents complaining the workers smoked on the job, adding to the fire hazards.

According to notices posted on the website of the local government-owned company in charge of the project, the Jingan Construction Technology Supervision Co., the effort had already been cited for safety violations. Another notice said safety inspections would be carried out weekly to “guarantee work safety.”

Shoddy or unsafe construction is a common problem in China, even in major cities like Shanghai. Last year, a nearly finished 13-story apartment building in the city’s suburbs collapsed, killing one worker. Officials blamed excavated dirt piled next to the building for the collapse.

The apartment building that burned Monday was part of a compound built to help provide teachers and retired teachers with housing.

Now many have nowhere to go.

“Luckily I wasn’t at home that day, so I survived, but my home was burned out,” said an older lady whose apartment was on the 19th floor of the destroyed building. She would only give her surname, Zhao, fearing trouble with authorities.

“Where am I supposed to live? What will I have for the rest of my life? I don’t think anyone has any answers,” she said.

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Associated Press researchers Ji Chen in Shanghai and Xi Yue in Beijing contributed to this report.