The Pheu Thai Party's legal team on Friday filed a complaint on the slow progress in resolving the land-grabbing case related to Khao Paeng plots in Koh Samui, party spokesman Prompong Nopparit said.
The compliant was lodged at the office of Justice Minister Pirapan Salirathavibhaga.
"I want Pirapan to check on the lack of progress in the investigation as he has promised," Prompong said.
The main opposition party brought to light the suspicion for unlawful possessen of plots by Democrat excutive Niphon Promphan and Tan Thaugsuban, the son of Deputy Prime Minister Suthep Thaugsuban.
The Department of Special Investigation has launched its probe since June 10 but Prompong said the case was stalled.