All were at least moderately wealthy, with homes and ample savings accounts accumulated over a lifetime of work in order to enjoy a comfortable retirement. Most died soon after drawing up wills that named her as the sole beneficiary.

The lurid deaths, some spaced only months apart, have grabbed national headlines, prompting tabloids to call Ms. Kakehi the Black Widow, after the venomous female spider that kills its mates. But the case also struck a deeper nerve. If the police allegations are true, Ms. Kakehi preyed on one of the biggest fears of Japan’s rapidly growing legions of retirees, members of a postwar mega-baby-boom who enjoy the longest life spans on earth — but are terrified of having to spend all those silver years alone.

A former bank teller who looks more like an aging auntie than a femme fatale, Ms. Kakehi (pronounced Kah-kay-hee) met the men through the dozens of specialized dating services that have sprung up here catering to lonely older people. The police say she carefully screened her prospective partners, looking for relatively wealthy men left single by divorce or the death of a spouse. Once she found one, the police said, she showered him with romantic emails professing her love.

“The elderly are an easy target because they have all the money, but they are also so afraid of isolation,” said Hiroyuki Kurokawa, a novelist who published a crime thriller this year that foreshadowed the Kakehi case. “This incident exposed the vulnerability of our aging society.”

The Japanese news media have begun using the title of the novel “Gosaigyo,” or “The Second Wife Con,” to refer to the Kakehi case. Mr. Kurokawa said he had recently signed a movie deal.