The Khmer Rouge have been blamed for many horrors in Cambodia, but until relatively recently the regime's use of sexual violence has not been well known.
Now the UN-backed war crimes tribunal has been asked to examine a range of allegations covering their period in power in the second half of the 1970s.
The development comes ahead of next month's 'global summit to end sexual violence in conflict' being hosted by the British government.
Reporter: Robert Carmichael
Speakers: Nicholas Koumjian, international co-prosecutor at the ECCC; Theresa de Langis, researcher on crimes of sexual violence; Zainab Hawa Bangura, UN Secretary-General's Special Representative on Sexual Violence in Conflict.
CARMICHAEL: he international prosecutor at Cambodia's war crimes court, Nicholas Koumjian, has asked investigating judges to examine allegations of serious sexual violence related to the court's fourth and final case.
That enquiry, known as Case 004, is already investigating three district-level Khmer Rouge chiefs for an array of serious crimes allegedly committed in the areas they ran between 1975 and 1979. Now Koumjian wants it to assess crimes of sexual violence too.
KOUMJIAN: "And these include forced marriages and also, within the forced marriages, individuals were required to have sex - so that amounts to rape within forced marriages - and also rape and other sexual violence outside of forced marriages. What we ask is that the investigating judge look at the evidence regarding that and the possible links to suspects in the case."
CARMICHAEL: Among the crimes in that category were instances where women were raped by Khmer Rouge cadres prior to being executed, as well as other occasions where women who reported they had been raped were then murdered.
The Khmer Rouge's rule of Cambodia is believed to have cost two million people their lives, most from execution, starvation, disease and overwork. And although the UN-backed court has over the years investigated an array of serious crimes, including genocide, torture and mass killings, it hasn't looked closely at crimes of sexual violence.
The exception to date has been for the crime of forced marriage, a state policy in which hundreds of thousands of couples were ordered to wed and often forced to consummate their union under threat of execution. The two Khmer Rouge leaders currently on trial are facing charges of rape but only within the context of forced marriage.
The reason for that? Because the investigating judges decided in 2010 that the Khmer Rouge's Moral Rule Number 6, which seemingly barred offences of a sexual nature, meant the leaders had a policy to punish crimes of sexual violence committed by cadres. Therefore they ought not to be charged with any breaches.
But researcher Theresa de Langis says studies have shown the moral code was not an anti-rape policy, but was used as part of the Khmer Rouge's exertion of complete control over every aspect of the enslaved population's lives.
DE LANGIS: "So the first thing is that Code 6 was not an anti-rape policy, and I really can't stress that enough. This was a policy that punished most often consensual sexual relationships, and in instances of rape, most often the victims."
CARMICHAEL: As more witnesses come forward after nearly four decades of silence, says de Langis, it's increasingly clear that sexual violence was widespread.
DE LANGIS:"And now we have a mounting body of evidence to say, absolutely, rape was used throughout the country, and one of its typical forms was very systematic or at least methodical rapes, gang rapes of groups of women who were already determined to be enemies, raped before they were executed."
CARMICHAEL: The tribunal's belated efforts to include these wider crimes of sexual violence in Case 004 come at a time of heightened international awareness of the issue.
Zainab Hawa Bangura is the UN Secretary-General's Special Representative on Sexual Violence in Conflict. She says resolutions passed by the UN Security Council in recent years are one reason the subject has moved from the margins to the mainstream.
She also credits efforts over the past two years by UK Foreign Secretary William Hague. Next month the British government will host a key summit on sexual violence in conflict.
BANGURA: "And now we have 145 countries who have signed a commitment to fight against sexual violence, and we are going to have the summit in June. I think all of this put together has created the momentum that everybody realizes that this is a crime, you commit it you will be punished, wherever you are, wherever you come from they will go after you."