In an exclusive, rare interview with VOA Khmer in July, Ta An Ta An denied overseeing major purges during the Khmer Rouge period.
According to prosecutors, Im Chaem took part in leading a purge of the Northwest Zone beginning in June 1977.
Between 500 and 700 Khmer Krom were killed at the site, according to the Documentation Center of Cambodia.
Victims of the regime had complained they cannot properly file as civil parties without knowing which sites are under investigation.
Questions over the handling of cases 003 and 004 have divided the Khmer Rouge tribunal (ECCC).
The court has completed one trial, of Khmer Rouge torture chief Duch, and is preparing for trial of senior leaders.
Rob Hamill's quest for justice is the subject of a new documentary, “Brother Number One”.
The international investigative judge for the court told VOA Khmer he anticipates at least two years of proceedings.
Members of the CPP have ignored a court summons for testimony from the former international investigative judge.
Thet Sambath said aspiring filmmakers who wish to do similar projects must be ready to work hard.
Some regime leaders have said in the past they had hoped to protect the country from Vietnamese occupation and others.
Victims from the Trapaing Thmar dam project say they want the judges to investigate closely Case 004.