Shares of Sino-Thai Engineering and Construction Plc (STEC) fell almost 20% on Wednesday after the anti-graft agency said executives at the company played a role in bribing state officials over the construction of a power plant by a Japanese company.
Sino-Thai and Japan's Mitsubishi Hitachi Power Systems Ltd (MHPS) were contracted to build a power plant for a unit of Electricity Generation Pcl in 2013.
Investigations by the National Anti-Corruption Commission (NACC) began in 2017 about alleged corruption involved in the project.
Sino-Thai, whose shares closed at 14.40 baht after losing a fifth of their value, could not immediately be reached for comment and it did not respond to a Reuters email. Executives and their lawyer could also not immediately be reached.
MHPS also could not immediately be reached for comment.
NACC secretary-general Voravit Sukboon said in a statement that top Sino-Thai executives "assisted in the wrongdoing of state officials, who demanded bribes from the Japanese company."
"The commission concluded that four state officials demanded payment of 20 million baht from a Japanese construction company to allow the use of a dock to transport equipment for construction of a power plant," Mr Voravit said.
The NACC said it had sufficient evidence to show the Sino-Thai executives violated the criminal code on bribery.
Four state officials could be relieved of their duty or face disciplinary action, the NACC said, adding that it would submit its report to the attorney general to adjudicate the case.