November, 1965: A suspected communist sympathiser is questioned at gunpoint in Jakarta. At least 500,000 were killed in a wave of violence that targeted communist sympathisers amid a power struggle between Suharto and President Sukar(TIME)

The men bound the thumbs of dozens of suspected communists behind their backs with banana leaves and drove them to a torch-lit jungle clearing. As villagers jeered, the prisoners were killed, one by one.

"There was no resistance," remembered Sulchan, then 21 and the deputy commander of an Islamic youth militia. "All of them had their throats cut with a long sword."

Sulchan was a killer in one of the worst atrocities of the 20th century, in which up to half a million people were massacred in 1965-66 in a purge of communists backed by the United States government. The bloodbath swept into power the dictator Suharto, who ruled for three decades.

In a series of interviews, Sulchan and three other killers said the massacres were in fact a carefully planned and executed state operation and described some of its horrors for the first time. All the men spoke of what they did with detachment and often pride, and expressed no regret at defending their country and religion, Islam.

The CIA refuses to talk about the operation even today, citing security reasons. But documents released by the National Security Archives in Washington show that the US Embassy passed the names of dozens of Communist Party leaders, and perhaps many more, to the Indonesian army. Documents also show that officials from the US Embassy in Indonesia passed on information to Washington about the killings of 50 to 100 people every night.

Even after Suharto's death in January, many who aided the purge are still in positions of power or influence, experts said. And the suppression of information about the abuses of the era means there has been no redress for the families of the dead.

"In all the newspapers published since late 1965, it is extraordinarily rare to find a perpetrator's description of the killings," said John Roosa, a professor at the University of British Colombia who wrote the book "Pretext for Mass Murder."

The four men give a glimpse into how the killings unfolded.

The frenzy began shortly after September 30, 1965, after an apparent abortive coup in which six right-wing generals were murdered and dumped in a well near the capital, Jakarta. Suharto, an unknown major general at the time, stepped into the power vacuum. He blamed the assassinations on Indonesia's Communist Party and claimed they were targeting Islamic leaders.

No conclusive proof of communist involvement in the coup has been produced, but the party was then the largest outside the Soviet Union and China, with some 3 million members. It also had an armed wing and financial clout. Its ties with China and Russia worried Washington, at a time when fears of communist takeovers in Southeast Asia were running high.

The four men interviewed were members of the local Islamic youth militia, Banser, or of anti-communist youth movements in East Java. They were in their 20s at the time.

Sulchan, now 64 and a preacher, said the "order to eliminate all communists" came through Islamic clerics with Indonesia's largest Muslim organisation, Nahdlatul Ulama. Sulchan led the first killing in his neighbourhood—that of a teacher, Hamid, said to have had communist ties.

We "hit him in the head with a sledgehammer, and he died instantly," said Sulchan.

On one night, Sulchan's platoon helped unload 20 to 30 prisoners at the execution site and beat to death a man who tried to escape. The rest were forced to the ground and killed. A man pleaded with his executor to tell his child to study the Quran, Islam's holy book. The executor agreed, then murdered him, too.

The bodies were dumped in a ditch. Such scenes were repeated across Java, Sumatra and the eastern island of Bali for several months in 1965 and 1966.

"I am convinced the actions were justified because communists were the enemy of my religion," said Sulchan. "I thought: 'This is what people get for not submitting to religion.'"

Sulchan's superior, Mansur, commanded the Banser militia for two-years and describes a highly efficient operation. Mansur, who like many Indonesians goes by one name only, collected names of suspected communists in the region. Their houses were marked in red on maps, and he ordered his men to round them up.

Those who resisted were killed on the spot. Others were taken to detention centres, then trucked to killing fields and shot, stabbed, beheaded and beaten to death, he said. He saw the slaying of hundreds of unarmed detainees in his village.

"We didn't want the country to become a communist state," said Mansur, sitting on a porch bench after returning from Friday afternoon prayers. "I don't have any regrets."

Even today, a ban on the Communist Party remains in force in Indonesia, and people marked as ex-political prisoners endure lingering mistrust and discrimination.