Drinking water with 1,600 times the acceptable level of chlorine was served to three infants at a nursery school in Tokyo earlier this week, but none was injured, Koto Ward officials said.
The nursery school decided to file a complaint with the police because it suspects someone intentionally spiked the water.
According to the Koto Ward officials, three pupils in a class for infants under 1 year old were given warm water from a pot late Monday afternoon and one of them spit it out.
The hot water was later found to contain chlorine at a concentration of 1,600 parts per million, the officials said. The Tokyo Metropolitan Government's standard for chlorine in drinking water is between 0.1 ppm and 1 ppm.
The water was also served during lunch and earlier in the afternoon on Monday, but nothing unusual was noted, the officials said.
The nursery's workers use liquid mixed with a diluted chlorinated compound to disinfect toys and desks. A concentration of 50 to 100 ppm is used for baths used to disinfect students before they enter school swimming pools. The compound is stored in a room at the facility, the officials said.