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His first memory is an execution. He walked with his mother to a wheat field, where guards had rounded up several thousand prisoners. The boy crawled between legs to the front row, where he saw guards tying a man to a wooden pole.

Shin In Geun was four years old, too young to understand the speech that came before that killing. At dozens of executions in years to come, he would listen to a guard telling the crowd that the prisoner about to die had been offered "redemption" through hard labour, but had rejected the generosity of the North Korean government.

Guards stuffed pebbles into the prisoner's mouth, covered his head with a hood and shot him.

In Camp 14, a prison for the political enemies of North Korea, assemblies of more than two inmates were forbidden, except for executions. Everyone had to attend them.

The South Korean government estimates there are about 154,000 prisoners in North Korea's labour camps, while the US state department puts the number as high as 200,000. The biggest is 31 miles long and 25 miles wide, an area larger than the city of Los Angeles. Numbers 15 and 18 have re-education zones where detainees receive remedial instruction in the teachings of Kim Jong-il and Kim Il-sung, and are sometimes released. The remaining camps are "complete control districts" where "irredeemables" are worked to death.