Inokashira Park Zoo in Tokyo, Japan, has launched an investigation after 31 of its 40 squirrels died over the course of a week, potentially the result of an accidental poisoning.

According to a statement released by the zoo on December 11, keepers had given all 40 squirrels an anti-parasitic medication on December 4, alongside spraying the squirrel cages with insecticide, as part of its sanitary measures.

It wasn’t long after the squirrels were returned to their cages, however, that one of the common Japanese squirrels fell ill and subsequently died. Over the course of the next week, several other squirrels followed, and by the morning of December 11, there had been a total of 31 recorded fatalities.

“We are currently investigating the cause of death and monitoring the progress of the surviving individuals. We sincerely apologize for any concern this may cause,” said the zoo. “We will endeavor to determine the cause of death through pathological tests and other means, and will notify you as soon as the facts are known.” Specimens are being sent to external research institutions as part of the investigation.

Whilst the statement said that the three drugs used on the squirrels had been used in the past and according to instructions, the zoo also admitted that “the possibility of poisoning from the drugs cannot be ruled out.”

In the meantime, the zoo confirmed that the squirrel facility has been cleaned and confirmed safe, and while under observation, the remaining squirrels have since returned to display. The zoo is home to multiple squirrel species, including the Japanese squirrel, Japanese flying squirrel, and Japanese giant flying squirrel – one even went viral back in 2018.