By Guy De Launey
BBC News, Phnom Penh
Ta Mok in 1998. He took vital evidence to his grave.
The death of Ta Mok, one of the main leaders of Cambodia's brutal former Khmer Rouge regime, has confirmed the fears of long-time proponents of the country's genocide trials.
All along they have warned that delays in setting up the process would result in key defendants dying before they reached court.
Now the trials will have to go ahead without one of their most important figures.
Some Khmer Rouge-watchers think that Ta Mok could have been the key to understanding what happened in Cambodia between 1975 and 1979.
Nine years on
As military commander, he was behind the Khmer Rouge forces' drive through the country, which eventually brought Pol Pot to power.
Cambodian officials are sworn in for the Khmer Rouge tribunal
He was also a member of the political elite, however, establishing the ideology that eventually led to the deaths of almost two million people.
Ta Mok's courtroom testimony could have provided compelling evidence to link the atrocities of the Khmer Rouge era to the surviving former senior leaders.
Instead it seems that he died without giving prosecutors any meaningful information.
Nine years have passed since Cambodia and the United Nations agreed to work together on the trials, and it is almost three decades since the Khmer Rouge fell from power.
Yet local and international prosecutors only began work at the start of July, with the court proceedings themselves still many months away.
Now the question is whether the remaining possible defendants will ever see the inside of the venue known as the Extraordinary Chambers.
They are all elderly and some have been in ill health.
"What we are afraid of is that one by one the former senior Khmer Rouge leaders will depart from our world," said Kek Galabru of the Cambodian human rights organisation, Licadho.
"At the end, the tribunal will face difficulties in finding key people to prosecute. When they die one by one, little significance will be left for the tribunal."
Another concern for victims' groups is that the former leaders will leave the country to avoid trial.
Apart from Ta Mok, all of them have been living freely in Cambodia, but recently there have been indications that they may be considering their options.
The former head of state, Khieu Samphan, has reportedly sold his house in the Khmer Rouge stronghold of Pailin, and the former foreign minister, Ieng Sary, has been making regular visits to Thailand.
The grave of Pol Pot in Anlong Veng, seen in a 2001 photo
Campaigners want the authorities to act.
"What are we going to do about this? Put some limits on their travel movements?" asked Youk Chhang of the Documentation Centre, which has been gathering evidence for the trials.
"[It is important] to show some kind of measures so that victims can have some kind of trust in the process. This is a bad beginning, having Ta Mok die in such a way."
The organisers of the trials have called Ta Mok's death "regrettable", but have refused to admit that it could damage the judicial process.
One official suggested it would make "no difference".
Some of the survivors of the Khmer Rouge are equally optimistic.
Theary Seng is the author of Daughter Of The Killing Fields, a book about her family's experiences.
"Ta Mok's death is devastating, because we have lost that evidence. But he is not the only potential defendant - there are thousands of them. Am I concerned that we are going to run out of individuals to put in the dock? No."
The fact remains, however, that the trials will only prosecute those "most responsible" for the deaths of an estimated 1.7m Cambodians, and the number of remaining senior figures is diminishing steadily.
Furthermore, only one likely defendant is currently in custody: Comrade Duch, the former head of the S21 prison where 14,000 people died.
His health has also been a matter of some concern.
Even though there is widespread support for the trials among Cambodian people, there is also a large degree of scepticism over whether they will prove to be a meaningful event.
The survival - and attendance - of the remaining leaders is crucial to the credibility of the process.
Age caught up with Ta Mok before justice.
Many Cambodians are hoping the same does not happen to the other leading Khmer Rouge figures.