Chisako Kakehi is seen in a 2014 photo after her arrest on suspicion of poisoning her husband with cyanide in the "Black Widow" case in Japan. JIJI PRESS/AFP via Getty Images

A Japanese death-row inmate, dubbed the "Black Widow" after she used cyanide to kill her elderly lovers, has died in a detention center at age 78, officials said on Friday.

Chisako Kakehi was sentenced to death for the murder of three men, including her husband, and the attempted murder of another man about a decade ago in a case that gripped Japan.

Her death was confirmed at a hospital on Thursday after she was found lying in her cell at the Osaka detention center, a justice ministry official told AFP.

The cause of death has yet to be determined, he said. Japanese media said it could be from an undisclosed illness.

Kakehi's death sentence was upheld in 2021, with Supreme Court judge Yuko Miyazaki saying she had "used cyanide on the men after making them trust her as a life partner."

"It's a calculated, cruel crime based on a strong intent of murder," Miyazaki said.

Kakehi reportedly amassed one billion yen ($9 million at the time) in insurance payouts and inheritance over 10 years but subsequently lost most of the money through unsuccessful financial trading.

She had relationships primarily with elderly or ill men and met some through dating agencies, where she reportedly stipulated that prospective partners should be wealthy and childless.

The poison was found in the body of at least two of the men she was involved with and police reportedly found traces of cyanide in the rubbish at her Kyoto home.

Her partners' deaths weren't investigated immediately because police initially determined they died from illnesses, with no autopsies being performed on most.

Her arrest only came after police discovered her most recent husband, 75-year-old Isao Kakehi, died from cyanide poisoning. Police then began looking into the earlier cases and found a pattern.

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