A court official read the documents Tuesday in the trial of three former top leaders of the Khmer Rouge, the Phnom Pehn Post reported.
Nuon Chea, Khieu Samphan and Ieng Sary are on trial for their roles in the deaths of former Lon Nol civil servants, police and soldiers in a bid to create a homogeneous society.
The document said the provincial officials were summoned to an assembly in April 1975 where members of the Khmer Rouge told them they would be integrated into the new government. However, the officials were later taken into the countryside in groups of 30 and 40, their hands tied behind their backs, shot and buried in their uniforms in mass graves.
The evidence showed the three defendants had an understanding of the policy of targeting specific groups.
Another party document revealed in court said only two classes of people -- peasants and workers -- would be allowed to exist in the new order.
Nuon Chea was scheduled to testify Wednesday about the killing field site.