Japan has been gripped by the sensational case of a millionairess who amassed her fortune by allegedly killing her husbands and romantic interests over the course of 20 years.

Chisako Kakehi, 67, whose assets come to JPY8 million (over US$65 million), was taken into custody in November after her fourth husband, Isao Kakehi, was found to have died of cyanide poisoning.

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This follows the deaths of six other men tied to Ms. Kakehi: Her first husband died in 1994 at the age of 54; her second husband passed away in 2006. A third husband died at 74 in 2008. A boyfriend, believed to have cancer, passed away in 2009, and in 2012 her then-fiance collapsed while riding a motorbike and died. Traces of cyanide were found in his blood, reports say.

In September of this year, Kakehi's 75-year-old boyfriend also died, again with cyanide found in his system.

Ms. Kakehi married her latest partner two months before the husband suddenly fell ill and passed away at their home in the city of Kyoto. Reports say he willed JPY1 billion (US$8.5 million) to his new wife, and it was his death that launched the investigation.

The former bank worker denies any involvement in the deaths, and police admit that some of the deaths appear to be of natural causes.

"We can't say how many now. Given their advanced age, we have to proceed carefully to judge whether their deaths were actually the result of foul play or not," an investigator involved with the case told the AFP news agency by phone.

Upon Ms. Kakehi's arrest, police found medical equipment used for the administration of drugs at another apartment she kept in the south of Kyoto. 

Japanese media quickly dubbed Ms. Kakehi a "black widow," a female type of serial killer who specifically targets and murders romantic partners for material gain. While female serial killers -- those murders who kill three or more victims -- are rare (American criminal pathologists estimate only one out of six serial killers are women), Japan has seen a sudden spike in "black widow" murders. 

In 2012, Kanae Kijima was sentenced to hang for the murders of three men, aged 41, 53 and 80, whom she met through Internet dating sites. Another woman, Miyuki Ueta, is currently appealing her death sentence for her involvement of the deaths of two men.

Japanese investigators found Ms. Kakehi kept her assets in up to 10 different accounts under several different names. 

The Daily Yomuri newspaper stated Ms. Kakehi met her partners through marriage and dating agencies from all over western Japan, a common practice in the country. However, she used different names for each, and stipulated wanted the man to be "elderly," "living alone," and "with certain assets." The Yomuri goes on to say she kept several accounts open even after she married.

Police suspect Ms. Kakehi was searching for another man when she was taken into custody. 

TagsBlack Widow, husband killer, serial killer, Chisako Kakehi