Cambodia's genocide tribunal formally indicted a former prison chief of the country's notorious Khmer Rouge on Tuesday, paving the way for a historic trial.

The U.N.-assisted tribunal said in a statement Tuesday that its investigating judges issued the indictment upon ending their investigation of Kaing Guek Eav — also known as Duch — whose Phnom Penh prison was used as a torture center.

Duch, charged with crimes against humanity and war crimes, is the first suspect to be indicted by the tribunal. He and four other former senior members of the Khmer Rouge, who held power in the late 1970s, were taken into custody last year.

The radical policies of the communist group are considered responsible for the deaths of some 1.7 million people from starvation, disease, overwork and execution. No senior member of the group has ever stood trial for the atrocities.

The tribunal's announcement marks another "important moment in the history of the court," said Peter Foster, a spokesman for the U.N.-assisted tribunal.

He said the indictment sets the stage for the first trial of the tribunal, which began its work in early 2006. No date has yet been set for a trial, but tribunal officials have previously said it was expected to begin in late September.

Duch, 66, headed S-21 prison, the Khmer Rouge's largest torture facility, which used to be a school and is now the Tuol Sleng Genocide Museum. About 16,000 men, women and children are believed to have been held there. Only 14 are thought to have survived.

When Duch was detained by the tribunal in July last year, he was charged only with crimes against humanity, with the war crimes charge being added with the end of the investigation against him.

Duch will be tried by a panel of five judges — three Cambodian, one French and one New Zealander — according to a 2003 pact between Cambodia and the United Nations establishing the tribunal.

The other four suspects being held by the tribunal are former top lieutenants of late Khmer Rouge leader Pol Pot, who died in 1998. They are former head of state Khieu Samphan, former chief ideologist Nuon Chea, ex-Foreign Minister Ieng Sary, and his wife Ieng Thirith, who served as the Khmer Rouge social affairs minister.

They also face charges of crimes against humanity and war crimes.

Cambodian politics and disagreements between the government and the U.N. delayed the establishment of the tribunal for years. Its work was further delayed by disagreements among judges over the procedural rules and controversies involving allegations of kickbacks among Cambodian staffers.

The tribunal, which is mostly funded by donations from foreign donors, is facing a budget crunch. The $56.3 million that was originally earmarked proved inadequate because the tribunal has had to recruit more staff and expand its work.

A revised budget estimated the cost of carrying out the tribunal's work through 2010 to be $143 million. The tribunal is $86.7 million short of that goal.