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(NEXSTAR) — Health officials in Germany are trying to track down thousands of people whom a nurse may have injected with saline instead of the COVID-19 vaccine.

The unidentified nurse is believed to have administered the bogus shots at the Red Cross vaccine clinic in Friesland, Germany between March and April of this year, according to District Administrator Sven Ambrosy.

Wilhelmshaven police are currently investigating the extent of the unidentified nurse’s alleged actions.

“Today I had the sad duty to inform around 8,600 people who may have been affected that it cannot be ruled out they may have received a saline solution instead of their vaccination at their vaccination appointment. For peace of mind we would recommend people get an additional vaccination,” Ambrosy wrote on Facebook.

The 40-year-old nurse initially admitted to injecting six people with saline, according to NDR News, and said she accidentally broke a vial with the Pfizer vaccine and was ashamed to tell the center. She was dismissed from her position.

Suspicions grew, however, after police spoke with witnesses and found that the nurse was openly critical of the vaccine on social media. The 8,557 patients who may have been affected are mostly over 70 years old, but the group also includes doctors and other health care workers who received shots during that same period.

Saline is not harmful and is also used in the preparation of the Pfizer COVID-19 vaccine injection, but it won’t protect against the virus on its own.

“The fact is, we do not know how many of them are unvaccinated or only partially vaccinated,” Heiger Scholz, who heads the Lower Saxony COVID-19 crisis team, told NDR on Tuesday. “It may be that only every third syringe was manipulated, it may be that there are no further cases. Unfortunately, the woman is not cooperative with the police, she is silent.”

Police investigator Peter Beer said at a news conference that investigators have “a reasonable suspicion of danger” after speaking with witnesses.

Health officials are now urging the thousands of people who may have missed one or both shots to get vaccinated again as soon as possible.