Known as the "Ruhr Hunter," Kroll operated in the Ruhr-area of West Germany for 21 years to the tune of 14 murders. Kroll had a poor memory, and in all likelihood, these were but a few of his many murders. Born born in Hindenburg, Oberschlesien, near the Polish border on April 17, 1933, "Jockel" committed his first murder at the tender age of age of 21, just 3 weeks after his mother's death. By the early sixties he already committed six murders. Most of his victims were young women and girls whom he raped and strangled. Kroll was sexually motivated in his attacks, but his cannibalism was mere expedience. Meat was expensive, so he hunted his own.

Kroll was arrested on July, 3, 1976, when 4-year-old Monika Kettner disappeared and a massive search was immediately launched. A neighbor who was bugged by Kroll's explanation of why their communal bathroom was clogged, complained to police about the incident. According to the man, Kroll told him the drain was clogged with "guts." When police plunged the toilet, they discovered a set of internal organs, along with pieces of flesh. Armed with a search warrant police entered Kroll's apartment to find plastic bags full of human flesh in the refrigerator as well as a stew simmering on the stove with carrots, potatoes, and the tiny hand of the missing four-year-old.

Kroll was an unwashed-looking, balding little man who kept to himself. The local children called him Uncle Joachim and could often be seen holding hands with him and playing. Their parents regarded Kroll as kindly and trustworthy. Kroll was unable to achieve a normal relationship with a woman, so he killed them first. He was highly sexed however, and when he was arrested police found in his room a well-used rubber sex-doll. After his crimes, sometimes during which he would ejaculate twice, he would come home and use the rubber doll until he was satisfied. Kroll also had a large collection of children's dollies which he used as bait fot the little girls he would befriend. He also used them to practice strangling children. Sometimes he would masturbate with one hand while strangling a doll with the other.

Kroll evaded capture for 21 years by varying his motive and type of victim. Also, Kroll appeared to be such an unremarkable little man. Several men were punished for his crimes until his capture. In 1979 the confessed Ruhr Hunter was charged with eight counts of murder and one of attempted murder. On April 8, 1982, Jockel was found guilty and sentenced to 9 life terms. On July 1, 1991, Kroll died of a heart attack in the prison of Rheinbach, near Bonn.