Only one building in Bangkok fell during the earthquake on Friday that rocked Myanmar, hundreds of miles away. Recovery efforts continue with at least 15 people killed and dozens still missing. Determining the cause could take months.

But interviews with workers who had been on the site, together with early official findings, highlighted potential problems with construction design and quality.

At the center of the scrutiny is China Railway 10th Engineering Group, a Chinese state-owned company with about a dozen other projects in Thailand and whose contractors tried to remove documents from the site after the disaster.

Behind that Chinese company is its parent, China Railway Group — a Chinese infrastructure giant with soaring debt, a hunger for new projects and subsidiaries facing accusations of weak safety in several countries.

Workers in Bangkok told The New York Times that China Railway 10th, which was part of a consortium constructing the building, underpaid contractors who turned to lower quality materials, and used columns narrower than usual.

Thai officials testing twisted metal from the ruins said they found substandard steel bars — made by a Thai factory with Chinese owners that the authorities had shut down in December.

An anti-corruption watchdog also said it had flagged construction irregularities at the 30-story tower before March 28, when fleeing workers watched the high-rise implode.

“The pillars on the third floor — where I stood and looked back — the beams didn’t burst,” said Netiphong Phatthong, 38, an electrician who barely escaped, waiting at the site for news about his missing friends on Tuesday. “They crumbled as the metals inside were squashed.”

Thai officials, describing the collapse as a blow to the country’s image, have moved aggressively to investigate the consortium with the $62 million construction contract for the tower. It had been meant to house government auditors.

At a cabinet meeting Tuesday, Prime Minister Paetongtarn Shinawatra ordered the authorities to also scrutinize all projects in Thailand involving China Railway 10th. She did not mention whether the parent company would be investigated.

“We really need to find an answer,” Ms. Paetongtarn told reporters. “We need to tell the people and the world what happened in Thailand.”

Under Pressure

The joint venture for the 30-story tower was registered in Thailand in 2018. It included a Thai developer, Italian-Thai Development, but according to workers, daily operations were run by China Railway 10th.

A promotional video that the Chinese company recently released showed drone footage of the building and praised the quality of its construction. China Railway 10th is also responsible for an airport terminal in southern Thailand that was supposed to be finished in January but was less than 40 percent complete as of March, according to the transport ministry.

China Railway 10th’s Bangkok office did not respond to emails seeking comment. The Chinese Embassy, in a statement on Facebook, urged Chinese companies to cooperate with the Thai government’s investigation.

China Railway Group, the parent company, got its start building most of China’s almost 30,000 miles of high-speed rail lines. But in recent years, as demand for new projects at home faded, the company and its subsidiaries have expanded their scope in a rush to bring in work.

Its timing often coincided with Chinese government priorities. In 2019, as Beijing sought closer ties in the Pacific, China Railway Group bought a dormant gold mine in the Solomon Islands. Workers there told The Times that safety concerns were often ignored.

Along the way, the company’s debt soared. Its 2024 annual report shows total liabilities worth $211 billion, almost double the $112 billion that the company carried five years ago.

Victor Shih, a specialist in Chinese politics and finance at the University of California, San Diego, said that when a company has such a heavy debt burden, “the pressure to generate cash flow to service debt can be quite intense.”

Another China Railway subsidiary has been blamed — along with allegedly corrupt officials — for the collapse of a train station canopy in Serbia last November, which killed 15 people.

Adding strain in Bangkok, construction was behind schedule. The building was less than half-finished despite a 2020 start. Mana Nimitmongkol, president of the Anti-Corruption Organization of Thailand, said the government had threatened to cancel the project in 2024 because of delays.

Adding suspicion, four Chinese nationals who identified themselves as subcontractors were caught on camera the day after the earthquake removing documents from an office behind the rubble. They told the authorities it was for an insurance claim. The police seized the documents; the men were detained, then released.

Weak Materials

Structural engineers in Bangkok struggled to understand why the building buckled all at once.

Pennung Warnitchai, director of Earthquake Research Center of Thailand, who helped draft Bangkok’s earthquake-resistant standards, said that the building should have been able to stay up. The ground motion detected in Bangkok after the earthquake struck, he said, “was about one-third to one-half of the level we consider in the design of typical buildings.”

Thailand generally follows an American model for earthquake protection, which means skyscrapers start with a reinforced concrete core. The core is typically a rectangular shaft in a building’s center, with floors and vertical support columns emanating outward.

In the case of the building that fell, the designers set the core off-center, said Mr. Pennung, adding that he saw the plans. Other buildings with a similar design did not collapse. But because the earthquake’s distance created longer periods between waves of motion, and with much of Bangkok built on soft soil, he said, the building may have both swayed and twisted, compounding the danger.

Mr. Pennung and several other engineers stressed it was too early to determine the cause of the collapse. Steel quality, design, work standards and soil could all play a role. But he said that evidence from workers and officials gathered so far, plus video footage of the quake, suggested that the core and supporting pillars or columns gave way on the lower floors.

“It seems that it collapsed from the bottom, not the top,” he said.

Other experts agreed that the building did not topple so much as drop, which corresponds with accounts from workers who escaped at the last minute.

Six employees from the downed building, who worked from the third to the 10th floors, said that the project often seemed unsafe. Some of the issues they reported were relatively minor: many workers wore sneakers rather than steel-toed boots; requirements for a harness on higher floors were ignored.

Three workers with experience on several lower floors said the columns of the building were thinner than those they had seen at other high-rise construction sites. Apichat Chaihlao, who worked on the eighth floor, said his supervisor was so concerned that he measured the columns himself “and he said these pillars are not right.”

“I didn’t think about it at that time, but now,” he added, “compared to the other projects I’ve worked on, that floor did not look sturdy.”

Two subcontractors, who declined to give their names out of fear of reprisal by their employer, said Chinese managers often disregarded suggestions from Thai counterparts and used lowball contracts that led to lower quality materials.

Reports about the China Railway Group’s subsidiary in Serbia also highlighted a lack of quality control. There, an engineer who had worked on the station with the fatal collapse said that a contractor hired by the Chinese consortium in charge had ignored design specifications and added extra concrete to the canopy.

Nonthichai Likitaporn, director of the Thai Industrial Standards Institute, said the problems with the Bangkok building included using steel that was too weak.

Samples of steel bars in two sizes that were collected from the site failed tests by the Iron and Steel Institute of Thailand for their mass, chemistry and ability to handle pressure. Other samples met the required standards, Mr. Nonthichai said, but the problematic metal was made by the same Chinese company: Xin Ke Yuan Steel Co., which had a factory in Thailand’s Rayong province.

Thai authorities closed the factory in December, citing risks to safety after an accident involving gas leaks. They seized more than 2,400 tons of steel after tests found that its rebar did not meet approved criteria for its ribbing height and boron content, undermining adhesion with concrete and weakening its strength.

The authorities also demanded that the company’s executives recall any steel already sold. It is not clear that they did that.

The company could not be reached for comment at the phone number listed on its website.

Chinese construction networks led by state-owned firms have played major roles in Bangkok’s recent building boom, adding apartments, train lines and other projects. The Thai government has increasingly close relations with China, which is Thailand’s largest investor. Last month, at Beijing’s request, Thailand deported 40 Uyghur asylum seekers back to China, drawing a sharp rebuke from U.N. officials and activists who had long warned that the men would possibly face torture and imprisonment upon their return.

Within China, Beijing has sought to censor coverage and discussion of the collapsed building. An explanatory story about the building’s structure by an investigative news outlet, Caixin, and a short piece by the official news agency, Xinhua, about the casualties caused by the collapse, were removed soon after they were published.

But in Thailand, fears of tall buildings are now common — and outrage has soared.

“Thailand’s public sentiment has been increasingly critical of Chinese business presence and government dealings,” said Thitinan Pongsudhirak, a professor of political science and international relations at Chulalongkorn University.

“The collapse of the Chinese-built building,” he added, “is likely to reinforce this critical view.”