The tigers' cage: inside China's Qincheng prison

The sound of a familiar cough, from a nearby cell he could not see, tormented Yan Mingfu through some of the darkest days of his seven-year confinement at the notorious Qincheng prison.

It was a muffled sound, perhaps just metres away, yet there was something about it that reminded Yan, who would go to become the head of the United Front Work Department, of his father.

Since all inmates of Qincheng were kept in solitary confinement, Yan was able to console himself only by reasoning that there would be little rationale for a retiree such as his father to be among those persecuted during the Cultural Revolution.

Yet the suspicion - shared in a recently published memoir - turned out to be true. Yan senior died only a few days after his son was disturbed by his coughing. Yan junior found out only after his release in 1975.

Watch: Exteriors of China's infamous Qincheng prison

Decades have passed since the two Yans were imprisoned within its walls, but Qincheng, a maximum-security prison designated for the detention of disgraced senior Communist Party officials, remains shrouded in secrecy.