Mr Somsak told police he was forced by Ms Pornchanok to tell police that his brother was not at Tanaka's shophouse the night he died.
Mr Somchai has confessed to murdering Tanaka and disguising it as an accident.
However, Pol Lt Col Chaiwat Thapanaphongphaibul, the investigator with Bang Phli district police station in charge of the original probe into Tanaka's death, would face an investigation to determine whether he distorted evidence so Tanaka's death was recorded as an accident.