A Japanese zoo has launched a probe after apparently massacring 31 of its 40 squirrels by mistake with treatments meant to kill parasites, officials have said.

Keepers at the Inokashira Park Zoo in Tokyo injected the animals with anti-parasitic medicine on Dec. 4 as part of a sanitary precaution, while also spraying insecticide over their nest boxes.

One of the bushy-tailed rodents — a common Japanese squirrel — died soon afterward and over subsequent days more and more perished, with 31 fatalities recorded by Monday morning.

"The possibility of drug-induced poisoning cannot be denied," the zoo admitted in a statement Monday. "We're currently investigating the cause of their deaths and observing the conditions of surviving individuals," they said, adding that a pathological examination of the corpses was underway.

"We offer our deepest apologies", the statement said.

The zoo insisted the drugs in question were used in proper doses and had been administered before.

The facility has been cleaned and vetted for safety, the zoo said, and the surviving squirrels were back on display.

Japan is home to three kinds of the tree-dwelling creatures: the Japanese squirrel, the Japanese flying squirrel and the Japanese giant flying squirrel.