A leading American war crimes expert has been named to advise the international tribunal trying former Khmer Rouge leaders in Cambodia.

U.N. Secretary General Ban Ki-moon has appointed David Scheffer as the U.N. “Special Expert” to advise the panel, which is currently trying the top three surviving Khmer Rouge leaders.

Scheffer, who served as ambassador at large for war crimes during the administration of former President Bill Clinton, played a role in establishing the Khmer Rouge tribunal as well as similar special courts to deal with atrocities in Rwanda and the Balkans.

His appointment follows months of controversy at the court, where an investigating magistrate and several staff members have resigned amid charges some cases were not properly investigated.

Scheffer served for four years with the U.S. delegation at the United Nations and later represented the United States at negotiations leading to the treaty that established the International Criminal Court at The Hague.

He recalls his years of efforts on behalf of international justice in a recently published book, “All the Missing Souls.”

In Cambodia, he will find a court that is deeply troubled over pressure from the government of Prime Minister Hun Sen to wind up its work with no further prosecutions beyond those under way.

Reserve judge Laurent Kasper-Ansermet recently joined the tribunal in place of German judge Siegfried Blunk, who resigned to protest government interference. He and fellow investigating magistrate You Bunleng had been accused of closing the investigations of two other cases without visiting crime scenes or interrogating key witnesses.

Since his arrival, Kasper-Ansermet and You Bunleng have argued publicly over how to proceed with the cases.

The Khmer Rouge is blamed for the deaths of up to two-million people during its brutal rule in the late 1970s.