This crime spree has sparked no manhunt, but instead, a national self-evaluation.
For weeks, a string of copycat poisonings has affected policemen, housewives, schoolchildren, and construction workers across Japan. Many of the acts have been random - convenience store drinks and sandwiches have been tainted with cyanide and arsenic.
The response to the poisonings has yielded some benefits in improved cooperation and communication among local governments. But in much the same way recent US school shootings have led Americans to question the drift of society there, the poisonings have unsettled deeply held convictions the Japanese have about their society and themselves.
"In the past, didn't we all share the trust that Japanese people were good people?" asks Tatsuo Inamasu, a sociology professor at Tokyo's Hosei University. "Didn't we basically trust each other because we were Japanese? That invisible trust and respect toward others has disappeared."
The first of this summer's incidents occurred in the eastern city of Wakayama in late July, when 60 festival-goers were affected and four died after eating curry spiked with arsenic. On Sunday police arrested a married couple in Wakayama on suspicion of insurance fraud and poisoning an acquaintance last fall. They also may be connected to the other poisonings in their city.
Since July, there have more than a dozen incidents, though the police will not specify the exact number. Some have been targeted and personal acts, including an offense by a student who mailed tainted drinks to her classmates, and another by a supermarket employee who doused the salad dressing at his company cafeteria with insecticide.
Some sociologists attribute the crime spree to frustrations born of economic uncertainty. Professor Inamasu cites changing values, rising individualism, and a loss of national purpose. Police also cite the work of imitators looking for attention. Many cases have similarly involved food or canned drinks on store shelves that had been punctured and resealed with glue.
But the response to these crimes has yielded some long-term benefits, prompting nationwide discussions on how to respond to emergencies, and plans for a manual to help local communities deal with them. In September, meetings between prefectures about the poisonings laid the foundation for greater communication among local governments.
Still, for one homemaker whose community had a poisoning scare, shopping will never be the same. "If it's a pack of milk, I look for needle holes," says Keiko Tanaka, a homemaker from Suita City in central Japan. "I've become more cautious," she adds. "I can't trust people anymore."
The trust that both Mrs. Tanaka and Professor Inamasu refer to is the special product of a homogenous society where social protocol governs everything from the daily exchange of greetings to the seasonal exchange of gifts.
This sense of community, called the "mura" or village mentality, is a vestige of Japan's rural roots and has been described as the heart of Japaneseness. Though it has been weakened by the country's aggressive industrialization, it is a tenacious survivor.
It lingers in big-city neighborhood groups that cooperate to keep their street clean, in community notice boards that dot urban neighborhoods, and in summer festivals that draw people out to carry the local shrine.
It contributes to a general sense of safety that allows parents to send children as young as 7 to school on the subway by themselves. In rural areas, it remains strong. Countryside neighbors still help each other rebuild their homes and lives after fires or natural disasters.
The mura mentality has given Japanese a sense of belonging, trust, and order. But the poisonings reflect the breakdown of those traditional ties, observers say, even as they further damage them.
The decline of community trust comes at the end of a decade that has already tested Japan's self-image and self-confidence. Since Japan's phenomenal economic growth came to an end in the early 1990s, political, social, and economic uncertainties have chipped away at the country's sense of ordered stability.
The 1995 Kobe earthquake left many Japanese disillusioned by the central government's molasses-like response. Later that year, a religious cult staged a gas attack on Tokyo subways, upsetting the cherished belief that Japan was a safe country.
A 1996 Health Ministry scandal involving contaminated blood products robbed Japanese of their faith that the powerful bureaucracy protected their interests when politicians didn't. Subsequent corruption probes at the Finance Ministry cemented that distrust.
This year, Japanese have watched recession continue to erode their hard-won economic success, and a rocket from Communist North Korea pierce the skies above them.
These events have weighed on the national psyche, but since they've played out on the national stage, most Japanese have felt some insulating distance from them. The poisonings have been a more direct challenge as they are happening on a local level.
Tanaka, who is pregnant, says these events have made her realize her child will grow up in a different kind of Japan. But she sees this crime spree as an opportunity for Japanese to think about the kind of future they want. In the meantime, she says, "I hope for the best."