As reported by Claire Corgnou, Yvan Keller formed his own business and began working as a landscaper and gardener shortly after his release from prison. As he had a friendly demeanor and did not appear threatening, he gained the trust of his customers, many of whom were elderly women who lived alone.

As reported by The New York Times, Keller later admitted to killing and robbing several dozen elderly women within a 40-mile radius of Mulhouse, France, which included victims in the neighboring countries of Germany and Switzerland, between 1989 and 2006. During his confession, Keller reportedly said he would break into the women’s homes and smother them with a pillow while they were sleeping. He would then allegedly steal their valuables and any cash they happened to have in their home.

The New York Times reports Keller was meticulous and did not leave any evidence suggesting forcible entry or that a crime had been committed. Therefore, authorities determined the women simply died of natural causes. However, the families of several victims were convinced foul play was involved. 

On March 12, 1994, 86-year-old Ernestine Mang was found dead in her bed. LeParisien reports her son, Germain, was immediately suspicious, as his mother was found lying on her back with her sheets and blankets arranged in an “immaculate” manner. When he contacted the medical examiner, he was told his mother died of natural causes.