A U.N.-backed genocide tribunal has added the charge of war crimes to its indictments against former Khmer Rouge prison chief Kaing Guek Eav.

The tribunal issued a statement Tuesday after completing an investigation of Kaing Guek Eav, who is commonly known as Duch.

Duch is one of five former Khmer Rouge leaders facing prosecution for their role in the regime's reign of terror from 1975 to 1979. 

He is expected to be the first former Khmer Rouge leader to face trial in September. Last year, Duch was also charged with crimes against humanity.

During the Khmer Rouge's reign, Duch was the commander of the its notorious S-21 torture center.

About 1.7 million people died of murder, overwork, and starvation under the Khmer Rouge.

At S-21, about 16,000 people were tortured and interrogated for being suspected enemies of the ultra-communist regime. Most were bludgeoned to death in a field on the outskirts of the capital. Less than 10 prisoners survived.

Some information for this report was provided by AFP and AP.