Defense lawyers continued to attack the Document Center of Cambodia, claiming its archival material presented to the U.N.-sanctioned genocide tribunal is biased.

Four former senior Khmer Rouge leaders remain on trial for their part in the brief but brutal Khmer Rouge government of 1970s.

The Document Center was started in 1995 as a field office for Yale University's Cambodian Genocide Program with U.S. State Department funding. In 1997 it became an independent non-government organization with funding from several countries, including the United States, Australia, Japan and Canada.

The trial of the four Khmer Rouge leaders continues after the tribunal, the Supreme Court Chamber of the Extraordinary Chambers in the Courts of Cambodia, turned down the appeal of another Khmer Rouge member, Duch.

Also known as Kaing Guek Eav, Duch is a former math teacher and Christian convert who ran the feared Tuol Sleng prison, also known as S-21, in Phnom Penh under the Khmer Rouge government.

Duch became the first high-ranking official to be sentenced by the Chamber in Cambodia's highly political genocide trials. In 2010 he was sentence to 35 years in prison, less five years for time already spent in custody, and appealed the sentence last March.

On top of turning down Duch's appeal, the Chamber increased the 69-year-old's sentence to life in prison.

The Chamber said on its Web Site that is had considered the high number of deaths for which Duch is responsible -- a minimum 12,272 lives -- along with the extended period over which the crimes were committed -- more than three years.

These facts "undoubtedly place this case among the gravest before international criminal tribunals," the Chamber said.

The fact that Duch wasn't "on the top of the command chain in the regime" doesn't mean he should get a lighter sentence.

"There is no rule that dictates reserving the highest penalty for perpetrators at the top of the chain of command," the Chamber said.

Duch's increased sentence last week was met with cheers as well as relief by victims and their families -- fearful it might have been reduced.

However, many more Cambodians await the outcome of the ongoing trial four former Khmer Rouge cadres who were "top of the chain of command."

Ieng Sary, 86, was the regime's minister of foreign affairs and Nuon Chea, 85, was the Khmer Rouge's main ideologist.

Chea was "Brother Number Two" to "Brother Number One," Pol Pot, who died in 1998. Pot fled the country after an invading Vietnamese army toppled the regime in 1979.

Khieu Samphan, 80, was the president.

Ieng Thirith, 79, wife of Ieng Sary and Khmer Rouge social affairs minister, has been absent from proceedings because of suspected advanced Alzheimer's disease.

Their lawyers have focused on the work of the Document Center.

Jasper Pauw, a lawyer representing Chea, questioned the veracity of material collated under the Document Center's Accountability Project and handed to the prosecution, The Phnom Penh Post newspaper said.

He also asked why it was necessary for the center to, in its own words, "illuminate chains of command" during the time of Democratic Kampuchea, as the Khmer Rouge called Cambodia.

"Because we want to know who actually killed two million Cambodians," center Director Youk Chhang said. "We want to know what happened in our history."

Pauw asked if, through the Accountability Project, the center was trying to implicate senior Khmer Rouge leaders in "alleged atrocities."

Chhang said the Accountability Project's purpose was to "gather historical information for the court in order to find the culprits who committed crimes during that period," the Post reported.

Arthur Vercken, defense lawyer for Samphan, asked Chhang about the potential fabrication of documents, including one deemed by the center to have be a fake, the Post report said.

Chhang said it would be "virtually impossible" to fabricate 1 million documents.