He was a model citizen. A hospital volunteer. And one of the most sadistic serial killers of all time. But few people could see the cruel monster beneath the colorful clown makeup that John Gacy wore to entertain children in his Chicago suburb. Few could imagine what lay buried beneath his house of horrors--until a teenaged boy disappeared before Christmas in 1978, leading prosecutor Terry Sullivan on the greatest manhunt of his career.

Reconstructing the investigation--from records of violence in Gacy's past,...