(CNN) -- Breaking news update published 11:40 p.m. ET:
Former Khmer Rouge leaders Nuon Chea, the former Deputy Secretary of the Communist Party of Kampuchea, and Khieu Samphan, the one-time President of Democratic Kampuchea, have been found guilty by the Extraordinary Chambers in the Courts of Cambodia (ECCC) of crimes against humanity. Both have been sentenced to life imprisonment.
Original story published 12:58 p.m. ET
Thirty-five years after the fall of Cambodia's genocidal Khmer Rouge regime, believed responsible for deaths of at least 1.7 million people between 1975 and 1979, only a single person has been brought to justice over one of the 20th century's great atrocities.
That will change Thursday, when two top leaders of the former Khmer Rouge regime will hear verdicts for their alleged crimes against humanity, in the first trial they face relating to their alleged activities in the 1970s.
The octogenarians in the dock are Nuon Chea, the former Deputy Secretary of the Communist Party of Kampuchea known as "Brother Number Two," and Khieu Samphan, the one-time President of Democratic Kampuchea, the Khmer Rouge's state, known as "Brother Number Four."
Prosecutors are seeking life sentences for the accused, who both deny guilt and are seeking acquittal.
The men were senior leaders in the Khmer Rouge regime, which ruled Cambodia between 1975 and 1979. During that time at least 1.7 million people -- about a quarter of the Cambodian population -- are believed to have died from forced labor, starvation and execution, as the movement ruthlessly executed its radical social engineering policies aimed at creating a purely agrarian society.
Who is hearing the case?
The charges are being heard in Phnom Penh in the Extraordinary Chambers in the Courts of Cambodia (ECCC) -- a special United Nations-backed tribunal that was formed in 2006 to prosecute senior Khmer Rouge leaders and other regime figures responsible for especially heinous acts.
The "hybrid" tribunal -- officially "an ad hoc Cambodian court with international participation" -- uses both Cambodian and international judges and staff employed by the U.N. in order to ensure the trials are conducted to international standards and to mitigate against the weakness of the Cambodian legal system.
Eight years on, the ECCC has delivered only one verdict.
In the ECCC's Case 001, Kaing Guek Eav, commonly known by his alias, Duch, was sentenced to life imprisonment following his 2010 convictions for war crimes, crimes against humanity, murder and torture. He was the commandant of the notorious Tuol Sleng S-21 prison in Phnom Penh, where more than 14,000 people died.
The verdicts on Thursday in the case known as 002/01 will be the first time that senior leaders of the regime have faced justice.
Who are the accused?
Nuon Chea, born in 1926, was Khmer Rouge leader Pol Pot's brother-in-law, and was considered his right-hand man and a key ideologist throughout the regime's reign of terror.
Trained in law in Bangkok, the 88-year-old was second-ranked in the Communist Party of Kampuchea (as the Khmer Rouge is officially known) and served a short stint as Democratic Kampuchea's prime minister.
Prosecutors described him as an extremist who "crossed the line from revolutionary to war criminal responsible for the deaths of hundreds of thousands of Cambodians," according to the ECCC.
Following the collapse of Democratic Kampuchea in 1979, he remained a leading Khmer Rouge figure in the years the movement operated as a rebel guerrilla force in Cambodia's west. He surrendered in 1998, striking a deal with the government that allowed him to live as a free man near the Thai border until his arrest in 2007, according to the ECCC.
In his final statement to the court, Nuon Chea admitted he carried "moral responsibility" for events during the period, but also affirmed his innocence, according to the ECCC.
"The CPK's policy and plan were solely designed to one purpose only, to liberate the country from the colonization, imperialism, exploitation, extreme poverty and invasion from neighboring countries," he said.
"The CPK's policy was clear and specific: it wanted to create an equal society where people were the master of the country ... The CPK's movement was not designed to kill people or destroy the country. My hope and wishes were betrayed by those who destroyed the movement."
Like many other Khmer Rouge leaders, Khieu Samphan studied in Paris, publishing his doctoral dissertation on "Cambodia's economy and industrial development." On his return home, he became a professor and then took on a senior government position before joining the Khmer Rouge rebels.
In 1976, he became the head of state of Democratic Kampuchea, and in 1987, years after the fall of Democratic Kampuchea, he replaced Pol Pot as the head of the Khmer Rouge after the former's retirement.
Throughout the trial, he expressed remorse for the suffering of victims, at one point offering Buddhist prayers for the souls of those who had died. But he repeatedly expressed his position that he was merely a figurehead, with no role in Khmer Rouge policy.
In his final statement, he expressed his view that the court was pre-determined to find him guilty. "[W]hatever I did was to uphold the respect for fundamental rights, and build a Cambodia that was strong, independent and peaceful," he said. "Those who will decide on my case have refused to take into consideration the truth, and now classify me as a monster."
What are the charges?
The charges they face relate to alleged crimes against humanity committed in the course of two forced mass population movements after the Khmer Rouge came to power, and the alleged large-scale execution of officers and officials of the previous Khmer Republic regime that was toppled by the Khmer Rouge.
The first phase of forced movement was the evacuation of Phnom Penh that began on 17 April, 1975, and led to between 1.5 to 2.6 million people being driven from their homes out of the city in all directions, mostly without knowing their final destinations, according to documents provided by the ECCC.
Prosecutors allege that about 20,000 deaths occurred during the evacuation -- half from execution and half from starvation and exhaustion -- and say the evacuees were subjected to systemic mistreatment, forced labor and enslavement in their new rural cooperatives.
The evacuees were vilified by the regime as "new people" -- those identified as urban-dwellers, intellectuals or associated with anything foreign.
The second phase of forced movements related to those from the central, southwest, west and east zones of the country, which took place from September 1975 through 1977. Some were "new people" evacuated from urban areas, some were allegedly associated with the former regime and others were members of the mainly Muslim Cham minority, according to the ECCC.
Prosecutors argue that, through their alleged role in the two phases of forced mass evacuations, the accused played a key role in a criminal enterprise, and were guilty of crimes against humanity including forcible transfer, murder and persecution.
The pair are also facing genocide charges related to alleged crimes against Vietnamese and Cham Muslims, as well as other crimes against humanity and war crimes, in a related ECCC trial -- 002/02 -- which began in Phnom Penh last month after the court decided to break up the indictments against the men into separate trials.
Time running out
The decision to split the case, said Lars Olsen, legal communications officer for the ECCC, was partly to make the lengthy, complicated proceedings more manageable, but also to try to reach a conclusion on some of the charges during the lifetimes of the accused and the victims.
Nuon Chea observed most of the trial from a holding cell due to poor health, while two co-accused did not make it through the entire trial.
Ieng Sary, the Khmer Rouge's co-founder and former foreign minister who was known as "Brother Number Three," died during the trial in March 2013, at the age of 87.
His wife, Ieng Thirith, the Minister of Social Affairs during the regime, also stood accused, but was eventually ruled mentally unfit to face trial.
"Brother Number One," the former General Secretary of the Communist Party of Kampuchea, Pol Pot, died in 1998 before he could be brought to justice.
The three who have stood trial to date may wind up being the only people to be ever face justice for Khmer Rouge crimes.
The ECCC is investigating two other cases, 003 and 004, involving five suspects who are yet to be officially named -- one of whom is dead, according to former ECCC investigator Craig Etcheson -- with no decision made as to whether to proceed with the cases.
These cases targeted those "most responsible" for the most serious violations of law, said Etcheson, and were focused on mid-level Khmer Rouge cadres who implemented the orders of the senior leaders.
Whatever the decision of the international co-investigating judge considering the matter -- the national co-investigating judge has recused himself from those cases -- the path ahead appeared complicated, he said.
"The government, and Prime Minister Hun Sen in particular, has voiced considerable opposition to Cases 003 and 004," he said.
"If the international co-investigating judge orders one or more of the suspects in cases 003 and 004 to stand trial, it is also expected that the national co-investigating judge will dispute that decision, and so the process will move to the pre-trial chamber for the adjudication of that dispute. Therefore we can expect that come what may, much litigation remains to be contested."
There were no plans to indict anyone not currently under investigation, said Olsen, adding the work of the tribunal was expected to continue until at least 2019.
What closure will the process bring?
Etcheson, a war crimes investigator who is president-elect of the group Genocide Watch, said that public opinion surveys had consistently shown that a large majority of the Cambodian public supported the ECCC process.
Most would expect life sentences for the accused, he said, and would likely "welcome that outcome as providing a measure of justice," he said, while acknowledging the criticism the court has come in for.
The ECCC's glacial pace in securing only one conviction in eight years of operation, at a cost of $200 million, has been widely criticized -- especially given the suspects were nearing the end of their natural lives.
Proceedings were halted for long periods when national staff went unpaid, and in a country where Prime Minister Hun Sen initially came to power as a Khmer Rouge battalion commander before defecting, accusations of politicization have dogged the process.
Theary Seng, a Cambodian-American human rights activist and lawyer who lost both parents to the Khmer Rouge and testified against Nuon Chea in ECCC proceedings, told CNN the process was "political theater."
Cambodians accepted that, while life sentences were not particularly satisfying outcomes when handed down to elderly men, it was all that could be delivered by the process. However, while Cambodians did not expect perfect justice from the tribunal, it had taken too long to put the senior regime figures on trial, and what had been delivered by the tribunal was less than acceptable, she said.
'An imperfect vessel'
Etcheson agreed the ECCC had "clearly been an imperfect vessel with which to deliver justice for the crimes of the Khmer Rouge."
Its two-headed nature had rendered judicial decision-making "cumbersome and occasionally contentious" and there was "an emerging consensus among legal analysts that this model should not be replicated elsewhere due to these inefficiencies."
"But under the prevailing international and domestic circumstances at the time the parameters of the court were negotiated, it was also clearly the last, best hope for genocide justice in Cambodia," he said.
Criticisms of the cost of the process seemed unfair, he said, as the expense and scope of the tribunal had been deliberately kept low.
"I would suggest that with upwards of two million people killed during the Khmer Rouge regime, a $200 million dollar tribunal works out to roughly $100 per murder victim. Surely that is not an excessive price to seek justice for such a monumental crime."
On the allegations of political interference in the process, he said Cambodian PM Hun Sen faced the difficult task of "not just of delivering a sense of justice for the crimes of the past, but also of maintaining social stability in the present, and mending the tattered threads of society."
"This difficult balancing act cannot be easy, though it is often rather blithely dismissed by critics as a simple act of craven political convenience.
"But based on everything I understand about Cambodia, there can be nothing simple about it. Perhaps we should not be so quick to judge the formidable task of rebuilding a destroyed people and nation."