A Thai construction tycoon and 14 others surrendered to police on Friday over a building collapse that killed nearly a hundred workers during a powerful earthquake in March, authorities said.

The partially built 30-storey State Audit Office tower in Bangkok was the only building to collapse from tremors emanating from the powerful 7.7 magnitude quake in neighbouring Myanmar. Rescue teams have recovered 92 bodies during a six week operation from the collapsed site. Four were still missing.

Premchai Karnasuta, president of Italian Thai Development, one of Thailand’s largest construction companies, and the 14 other suspects were affiliated with companies responsible for building design and construction, said Metropolitan Police Bureau deputy commissioner Noppasin Poonsawat. They have been accused of negligence and breaching construction regulations.

“Expert assessments revealed that the architectural design did not comply with ministerial regulations or meet the technical standards outlined in the terms of reference,” Noppasin said.

There were also irregularities in construction materials used, including substandard concrete and steel, he said, adding that investigators also discovered forged signatures in engineering documents.

A watchdog organisation flagged days after the quake that corruption may have been involved in the building’s construction. Initial tests of materials gathered at the site in March indicated the presence of substandard steel among the wreckage structure, according to industry ministry officials.

The 15 suspects have been brought to court for pre-trial proceedings and have denied all charges, Noppasin said, while two additional suspects would report to authorities on Monday.

An investigation was still underway over the causes of the collapse, one of the country’s deadliest.

Italian Thai Development has said it is cooperating with authorities.

Premchai was convicted and sentenced to more than three years in jail in 2021 for poaching protected species after he was caught by rangers at a jungle campsite in a wildlife sanctuary with carcasses of protected animals, including a black Indochinese leopard.