Several years before his 2003 arrest by Belgian police, French authorities already had a long criminal file on Fourniret. Expatica.com pointed out in a July 1, 2004, article that in 1987, a French court sentenced him to seven years in prison "for rape and indecent assault on minors." However, he was free after serving only two years for good behavior and because"of the length of time he had spent in custody."

AP's Paul Ames reported that following his release Fourniret moved to Belgium and took a temporary job in a school lunchroom. Over the years, he worked at a series of other jobs, yet was unable to hold a position for long. It is not unusual for a serial killer to have difficulty keeping a job. Searching for victims always takes precedence over the routine of every day work.

Lichfield reported that Fourniret had "a string of convictions for sexual assault and rape" dating back to the 1960s. Moreover, a July 6, 2004, Exaptica.com article suggested that he had been arrested, possibly for sexual offenses, on four separate occasions between 1996 and 2001 and released each time due to lack of evidence. Lichfield also stated that Fourniret confessed to murdering women and girls "between 1987 and 1990 and in more recent years," yet denied killing anyone between 1990 and 2000.

Fourniret claimed that during his alleged "active" periods, which added up to five years in total, he murdered 2 girls a year. If his story is correct then his total number of victims would be 10, which is 2 more than what he confessed to. Conversely, police believe that he murdered 2 girls a year over a 15-year period, which would bring the number of victims to approximately 30. The exact number of women he murdered may never be known, but some believe that there could have been up to 40 or more victims.

According to Lichfield, Fourniret targeted mostly virgins. He further claimed that when Fourniret went hunting and knew he would bring back a girl or woman, he would dig three-meter deep graves in advance. Some of the graves were dug on his property in the French Ardennes, which he bought shortly after his release from prison. The money he used to buy the property was allegedly stolen from one of his victims.

While Fourniret was serving time in a French prison, he met fellow inmate Pierre Hellegouache. Ames claimed that Pierre was involved in the "far-left militant group Action Directe, which was responsible for a series of bomb attacks in France during the 1980's." On July 3, 2004 Expatica.com reported that Fourniret learned that the group's "war chest" of some $30,000 worth of gold coins was being held by Pierre's wife, Farida. When Fourniret was released from jail, he tracked Farida down, stole the cache of money and then murdered her. Ames stated that the money was used to buy 32-acres of land in Donchery, which housed an 18th Century castle. Shortly after his confession, Fourniret and his wife led police on a search of his property to look for some of the bodies he buried there.