A Wisconsin health system said Wednesday that 500 doses of coronavirus vaccine that had to be discarded after they were left unrefrigerated now appear to have been deliberately spoiled by an employee.

Aurora Medical Center first reported on the spoiled doses on Saturday, and said they had been accidentally left out overnight by an employee at Aurora Medical Center in Grafton.

In a statement late Wednesday, Aurora said the employee involved “today acknowledged that they intentionally removed the vaccine from refrigeration.”

Aurora’s statement said they had fired the employee and referred the matter to authorities for further investigation. Their statement said nothing about a possible motive for the action, and health system officials didn’t immediately respond to messages seeking more information.

“We continue to believe that vaccination is our way out of the pandemic. We are more than disappointed that this individual’s actions will result in a delay of more than 500 people receiving their vaccine,” the statement said.

Aurora declined to provide additional information, but said they would provide more details Thursday.