Judges at the UN-backed Khmer Rouge tribunal said Monday they face a number of complications in preparation of the court’s second trial, the combined indictment of four senior leaders, which has stalled investigations.

The trial of Case No. 002 pits the court against alleged atrocity crimes suspects Nuon Chea, the chief ideologue of the regime; Khieu Samphan, its nominal president; Ieng Sary, foreign minister; and his wife, Ieng Thirith, social affairs minister.

“The investigation is not yet finished due to the complicities of the crime and difficulties for the crime which happened decades ago,” said Judge Kong Srim, president of the Plenary Session of the tribunal, in his opening remarks.

The tribunal is currently undertaking the relatively straightforward trial of Kaing Kek Iev, also known as Duch, the former head of the regime’s lethal Tuol Sleng prison.

Judges and prosecutors are meeting for the full session, their sixth, every day this week, and are expected to discuss civil party status, procedures for the Supreme Court and participation of civil parties in Case No. 002.

“The co-investigating judges are interviewing, investigating and delegating to experts to report on the demography, and they are considering the request for some more investigations,” said Judge Silvia Cartwright, deputy chairwoman of the Plenary Session.