Police have launched a formal criminal investigation into the case that Ji Zhongxing filed against security guards in Xintang village in Houjie township of Dongguan, in which he was allegedly beaten and maimed eight years ago.

Police confirmed that the investigation began in September. "But no details are available at the moment," police in Dongguan, Guangdong province, said on Tuesday.

Ji, 34, a farmer from Heze, Shandong province, was convicted of setting off an explosive at Beijing Capital International Airport in July. He was sentenced on Oct 15 to six years in prison by a Beijing court.