The three most senior surviving leaders of the brutal regime stand accused of some of the gravest crimes in modern history for their roles in up to two million deaths in the late 1970s.
Worried the octogenarians would not live to see a verdict, judges at a UN-backed court in Phnom Penh split their complex case into smaller trials, saving the most serious atrocities for later proceedings.
But seven months into their slow-moving first "mini-trial", concern is mounting that the court -- faced with a worsening funding crunch and fresh fears over the accused's health -- will be unable to finish the entire case.
"This is it, this is the trial. Nobody believes there's going to be a second phase," said Anne Heindel, a legal advisor to the Documentation Center of Cambodia, which researches Khmer Rouge atrocities.
Led by "Brother Number One" Pol Pot, who died in 1998, the communist movement dismantled modern society, abolished money and religion and forced the population to work in huge labour camps in a bid to forge an agrarian utopia.
"Brother Number Two" Nuon Chea, ex-foreign minister Ieng Sary and former head of state Khieu Samphan deny charges including war crimes and genocide.
The first trial segment focuses mainly on the forced evacuation of city people to rural work sites, listed as a crime against humanity.
Yet for most Cambodians, far greater horrors than that took place under the 1975-1979 regime, including torture, mass purges and forced marriages -- events the court may never shed light on.
"I'm not happy. The truth is not being revealed," said prominent Khmer Rouge survivor Chum Mey, 82.
His comments were echoed by Heindel, warning against settling for a "narrow judgement" that does not "tell the story of what happened under the Khmer Rouge".
"It would greatly diminish the legacy of this court," she said.
The prosecution has twice asked judges, so far in vain, to widen the scope of the first trial by including more crimes, such as the killings of thousands of perceived "enemies" shortly after the evacuations.
"We believe that it is important that if this trial is the only trial against the accused that it is viewed to represent the most horrendous and prevalent crimes committed," co-prosecutor William Smith said.
Their demands took on greater urgency after Ieng Sary, 86, who has heart problems, was hospitalised for five days for bronchitis last month, forcing hearings to be postponed for a week.
Since opening statements began in late November, the court has held trial hearings on 78 days. On Thursday it adjourned for almost a month.
The court's funding problems are another major obstacle, according to observers.
The tribunal, paid for by voluntary contributions from donor nations, has cost more than $160 million since it was set up in 2006 and faces a $22 million budget shortfall this year.
Observers blame much of the donor fatigue on the court's controversial investigations into possible new Khmer Rouge cases, which are dogged by claims of government interference.
So far, the troubled tribunal has completed just one case, sentencing a former prison chief to life in jail for overseeing the deaths of 15,000 people.
For the elderly accused currently in the dock, any kind of sentence would be "a death sentence essentially" meaning donors might have little appetite to spend money on further trial phases, Ieng Sary's defence lawyer Michael Karnavas said.
"The question from the donors' point of view is: is it good value? Probably not," he said.
Karnavas said he does not expect judges to expand the scope of the first segment of the current trial but added they would have to show "flexibility" to defence teams if they did, including recalling witnesses.
"I'd rather have a nice clean trial," he said. "You invited me to the party, now let's dance. But you can't be changing the rules in the middle of the party."