The 30-storey tower, still under construction, was to house government offices, but the shaking reduced the structure to a pile of rubble in seconds, killing at least 13 people and injuring nine.

It was the deadliest single incident in Thailand after Friday's 7.7-magnitude quake, with the majority of the kingdom's 20 fatalities thought to be workers on the building site and hopes fading for around 70 still trapped.

Sprawling Bangkok bristles with countless high-rise blocks, but none have reported major damage, prompting many to ask why the block under construction gave way.

"We have to investigate where the mistake happened," said Thai Prime Minister Paetongtarn Shinawatra, who has ordered a probe into the materials and safety standards at the construction site.

"What happened from the beginning since it was designed? How was this design approved? This was not the first building in the country," she told reporters on Saturday.

The development near Bangkok's popular Chatuchak market was a joint project involving China Railway No. 10 Engineering Group (Thailand) -- an offshoot of China Railway Group (CREC), one of the world's largest construction and engineering contractors.

- Questions raised -

Testing of steel rebars -- struts used to reinforce concrete -- from the site has found that some of the metal used was substandard, Thai safety officials said on Monday.

Industry Minister Akanat Promphan announced that a committee would be set up to investigate, saying one supplier of the steel had failed safety tests in December and may have its licence withdrawn. He did not name the supplier.

Professor of Civil Engineering at King Mongkut's Institute of Technology Ladkrabang Suchatwee Sunaswat said there were questions to be answered.

"We have to look at the design. At the beginning, how they calculate, how they design. And in the rescue mission, how they collect evidence at the same time," he told reporters on Saturday.

- Safety complaints -

The local partner in the project, Italian-Thai Development (ITD) offered condolences on Monday to quake victims but said it was "confident" the incident would not impact its other projects.

Beijing-owned building conglomerate CREC is one of the world's largest construction and engineering contractors, with projects in more than 90 countries and regions, according to its website.

The Bangkok construction collapse is not the first time CREC and its subsidiaries have come under fire after deadly incidents.

A tide of anger was unleashed at authorities in Serbia following the deaths of 14 people when a roof collapsed in November last year at a train station built by CREC subsidiaries -- largely focused on reports of alleged shortcuts made with building projects.

Roisai Wongsuban of the Migrant Working Group advocacy organisation said there have been a large number of complaints from migrant workers employed by Chinese companies in Thailand about lax safety standards and poor labour rights.

"For Chinese companies we can't see the human rights due diligence, to see if labour standards are being met," she told AFP.

"There is always a power imbalance between employer and employee."

Bangkok's construction boom is powered by an army of labourers, a large proportion of them migrant workers from Myanmar, toiling on hot building sites for low pay.

The Migrant Working Group has called on Thailand's labour ministry to hold the employers involved in the construction project criminally liable if they have failed to meet health and safety laws.

- China sensitivities -

AFP has asked China Rail No. 10 Engineering Thailand and CREC for comment but has not had a response.

An announcement celebrating the completion of the main structure at the Chatuchak construction site posed on China Rail No. 10's official WeChat channel was deleted soon after Friday's quake.

AFP archived the post shortly after the tremors hit but before the page was removed.

Local media said that four Chinese nationals were apprehended on Saturday for attempting to retrieve documents from the collapse site.

But China is the largest source of foreign direct investment in Thailand, injecting $2 billion into the kingdom in 2024, according to Open Development Thailand, and the government typically handles anything linked to Beijing with kid gloves.

Paetongtarn said an investigation into the collapse launched on Monday would not be "specific to one country".

"We do not want one particular country to think we are only keeping eyes on (it)," she said on Tuesday.

At a small shelter near the site on Monday, 45-year-old Naruemol Thonglek waited for news of her boyfriend, electrician Kyi Than, who was missing under the enormous mound of concrete and twisted metal being lifted by mechanical diggers.

"I'm devastated," she told AFP. "I've never seen anything like this in my entire life."