The contractors of the under-construction office tower that collapsed in Bangkok during last Friday’s earthquake apparently used substandard steel bars made by a factory that had been closed by authorities.

Samples of two different sizes of steel bars collected from the site of the State Audit Office building failed tests by the Iron and Steel Institute of Thailand.

The metal was made by a company whose factory had been shut for other violations since December,  the Ministry of Industry said.

Images of the steel bars shared by the ministry and local media displayed the brand “Sky”, made by Xin Ke Yuan Steel Co, which had a factory in Rayong province. Authorities closed the factory on safety grounds in December due to an accident and seized more than 2,400 tonnes of steel.

The 30-storey building was the only building to crumble in the Thai capital after the 7.7-magnitude earthquake that hit Myanmar on Friday. The collapse killed at least a dozen workers and trapped dozens more.

The skyscraper was being constructed by ITD-CREC, a joint venture between SET-listed Italian-Thai Development Plc and China Railway Number 10 Thailand Co.

China Railway Number 10, which could not be reached for comment, is also set to be investigated.

The viral images of the dramatic collapse of the building, which was about 45% complete, have raised questions about the design, construction and quality of materials used. An anti-corruption watchdog said on Monday that it had raised a number of concerns about the project.