Fears over the safety of school dinners in Japan have driven one local authority to install metal detectors in its kitchens.
Since September, there have been periodic outbreaks of intentional poisonings in schools. In Nagano, which held last year's winter Olympics, a bizarre series of incidents has been reported where nails and other foreign objects were found in meals.
To prevent a recurrence of the incidents, the Nagano municipal board of education last week installed two metal detectors in each of the three municipal cooking centres that provide meals for 68 primary and middle schools in the city.
The objects were found in bread, dumplings and soup served at six primary and middle schools in Nagano in December. Three nails and a strip of vinyl were found in school lunches served to handicapped children at a primary school. None of the children was injured.
The authorities said that it was unlikely that pranksters were to blame, since the school's food-storage rooms are locked except when meals are going in or out. Police are treating the cases as part of the nationwide series of copycat poisonings that have taken place since a 37-year-old woman allegedly put arsenic into a pot of curry. It was served at a local festival in Wakayama in July. Four people were killed and 63 others were taken to hospital.
The police have reported 41 poisoning incidents across the country since then, one of them fatal. In September alone, there were 25 cases. That number dropped to six in October and five in November.
The police said that 13 of the 41 poisoning cases occurred inside schools or research institutes. The remainder were apparently indiscriminate. On 12 occasions, vending machines were targeted and in another 11 cases poisoned drinks were placed on shop shelves.
The police said they have made some arrests.