The massacre occurred at the execution site of Tuol Po Chrey, and thousands of Lon Nol soldiers were killed after being promised new positions in the army, The Phnom Penh Post reported.
In October, Khmer Rouge Tribunal judges added the massacre to the trial of the three former leaders — Ieng Sary, Nuon Chea and Khieu Samphan — who are being tried for crimes against humanity.
Ung Chhat testified Tuesday about what he saw.
"The Khmer Rouge would not allow the next vehicle to leave for Tuol Po Chrey unless they saw the previous one, which had transported Lon Nol soldiers to Tuol Po Chrey earlier, return. Later, I saw the vehicle which transported Lon Nol soldiers earlier return empty," he said. "At that site at the Tuol Po Chrey fort, I saw the dead bodies were on the ground with their heads pointing north."
Chhat vehemently denied receiving orders to kill anyone at Tuol Po Chrey and said he was not involved, the Post reported.
The court is hearing more witness testimony today.
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