A nurse in Germany is accused of deliberately administering saline solution instead of COVID-19 jabs last spring.
Police fear that more than 8,500 people could have been affected in the scandal, though initially there were only six reports.
Police said they found anti-vaccine views in the nurse's social media posts.
"We then found three or four of her WhatsApp chats from April 21, right before the act, so we suspect that she did it," Wilhelmshaven police officer Peter Beer said.
The scandal was revealed by one of the employees at the centre who had seen the suspect filling six syringes of saline solution, which she allegedly defended as a mishap with a vaccination vial.
At least 9,673 people were registered as having been vaccinated during the period in question.
Among those vaccinated were employees from the nursing sector. Those affected are now being asked to get the vaccine again.
"There is no danger whatsoever from this repeated vaccination and therefore we can only advise everyone to get vaccinated again just to be on the safe side," said Claudia Shroeder, a member of the crisis intervention team.
For more on this story, watch the video in the media player above.