A medical expert told Cambodia's Khmer Rouge tribunal Monday that one of its four defendants is suffering from memory loss and may have Alzheimer's disease.

The report from geriatric expert John Campbell means that Ieng Thirith, the Khmer Rouge's former social affairs minister, may be found unfit to stand trial for her role in the brutal regime blamed for up to 2 million deaths in the late 1970s.

At minimum, 79-year-old Ieng Thirith will have to undergo additional testing, likely pushing back the beginning of testimony in the landmark trial well into next year.

The tribunal began hearing preliminary motions in the case against the four top Khmer Rouge leaders a month ago, but defense motions and technical issues have already pushed back the likely start of the actual trial.

The others on trial are 79-year-old Khieu Samphan, the nominal Khmer Rouge head of state; 84-year-old Nuon Chea, described as the regime's chief ideologue; and Ieng Thirith's 85-year-old husband, Ieng Sary, the Khmer Rouge foreign minister.

The charges against the defendants include religious persecution, torture and genocide linked to the deaths of as many as 2 million people between 1975 and 1979.

The trial is the showcase event for the U.N.-backed tribunal, which was created to demonstrate impartial justice and foster national healing.