Cambodia's Vann Nath, one of only seven survivors of a vast and notorious Khmer Rouge torture center, died Monday at age 66.

The human rights icon and artist was hospitalized late last month after heart problems and has been in a coma for days. His son-in-law called his death “a big loss for the history of Cambodia.”

Vann Nath was one of only a handful of people to emerge alive from Phnom Penh's infamous Tuol Sleng prison, where more than 12,000 people died in the 1970s under Khmer Rouge rule. He later became a leading advocate for victims of Khmer Rouge atrocities.

His widow, Kith Eng, blamed his lengthy illnesses, which included chronic kidney disease, on the torture suffered at Tuol Sleng. She told the Associated Press last month she believes her husband would have lived a long and happy life but for the year spent at the hands of his captors.

Vann Nath's 1998 memoir — A Cambodian Prison Portrait: One Year in the Khmer Rouge's S-21 Prison — is the only written account by a survivor of the prison.

S-21 was later converted to a genocide museum, where many of Vann Nath's paintings depicting torture adorn the walls.

News of Vann Nath's death comes as an international tribunal prepares to begin the long-awaited trial of the four most-senior surviving Khmer Rouge leaders — all charged with atrocities during the group's 1975-1979 rule. The defendants, including the nominal Khmer Rouge head of state, 79-year-old Khieu Samphan, face charges of religious persecution, torture and genocide in the deaths of as many as 2 million people.

Khmer Rouge leader Pol Pot died in 1998.

The tribunal is also deliberating an appeal by convicted war criminal Duch, the one-time chief of Tuol Sleng prison. Duch was convicted of war crimes and imprisoned earlier this year for 30 years — a sentence later reduced to 19 years because of time served in detention.

Tribunal spokesman Lars Olsen, speaking Monday, described Vann Nath as the survivor “who gave voice to victims” both through his advocacy at the tribunal and through his lifelong work at the Tuol Sleng museum.