Cambodia's United Nations-backed war crimes tribunal has ordered the release of Ieng Thirith, the aging sister-in-law of former Khmer Rouge leader Pol Pot.

On Thursday, the court said the 80-year-old – once dubbed the “First Lady” of the Khmer Rouge – is unfit to stand trial for genocide because of a degenerative illness, likely Alzheimer's disease.

The ruling, which upholds an earlier decision, found that all treatment options have been exhausted and that Thirith's sickness is “likely irreversible.”

She was charged with crimes against humanity, genocide, homicide, torture and religious persecution related to the 1975-1979 rule of the Khmer Rouge, which resulted in the deaths of some 1.7 million Cambodians.

Clair Duffy, a Khmer Rouge tribunal monitor working for the Open Society Justice Initiative says the ruling was expected and that an appeal is not likely, since the prosecution last week conceded Thirith was mentally unfit to stand trial.

She tells VOA that many Khmer Rouge victims will likely not be pleased with the ruling, and that makes it critical that the court clearly explain the rationale behind its decision to the public.

“I think the court really needs to make clear what the reasons are for this decision,” says Duffy. “It is actually a positive outcome for the court as far as applying international standards and recognizing that no matter what someone is charged with, they need to be afforded due process.”

The court statement said Thirith's release does not reflect a ruling on her guilt or innocence. But all charges against the former social affairs minister for the radical Communist movement have been “postponed indefinitely.”

“In saying that, they've left an opportunity for an extremely remote possibility that there's some medical breakthrough in terms of treatment of dementia or Alzheimer's disease – as in a cure for that illness. But obviously at this point that's a very remote possibility,” says Duffy.”

The tribunal is seeking justice for the legions of victims who died of starvation, execution or lack of medical care during the Khmer Rouge's reign.

Ieng Thirith and her three co-defendants are the most senior survivors of the regime's leadership. All defendants deny the charges. Pol Pot died in 1998.