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Cambodia's Khmer Rouge tribunal has dropped crimes against humanity charges against a former lower-ranking Khmer Rouge official, saying she was not high enough in the regime's leadership hierarchy.

Prosecutors in 2015 had charged Im Chaem, a district chief during the Khmer Rouge's 1975-79 reign, with crimes against humanity including murder, extermination and enslavement.

An announcement Wednesday by the U.N.-assisted tribunal said the investigating judges had dropped all charges in the case against Im Chaem because she was neither a senior leader of the Khmer Rouge nor one of the most responsible officials.

The brutal policies of the Khmer Rouge are blamed for the deaths of 1.7 million Cambodians.

The tribunal has already convicted two top Khmer Rouge leaders and the head of its prison system for serious crimes.