Family members say 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 suffering heart problems and has been in a coma for days. His son-in-law told the French news agency his death was “a big loss for the history of Cambodia.”

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

His 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.

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.

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.