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.
Both Mr Somchai and Ms Pornchanok have been charged with the premeditated murder of Tanaka
They were charged after Mr Somchai confessed to colluding with Ms Pornchanok in killing Shimato and dismembering his body before stuffing it into four sacks and dumping them in a canal in Samut Prakan on Sept 21.
Police began their search for Shimato after his son lodged a complaint about his disappearance.