Daniela Poggiali, a 42-year-old nurse working at a hospital in northeastern Italy, has been arrested for an ongoing investigation into the deaths of 38 patients who died during her shift.

According to the New York Post, the nurse may have injected the patients with a deadly dose of potassium chloride simply because they or their visiting friends and family annoyed her. Potassium chloride injections leave no trace in the body after a few days, making most of the nurse’s murders very hard to prove.

However, Poggiali has been taken into custody for the murder of a 78-year-old patient named Rosa Calderoni who died from a potassium chloride injection that was recent enough to prove the nurse’s guilt. Calderoni had been admitted to the hospital for a routine illness and died suddenly. Investigators ran tests after her death and found high traces of potassium, which can result in cardiac arrest.

The elderly woman’s death tipped off police that the other mysterious deaths may have been foul play, namely the work of Daniela Poggiali. Authorities will continue to investigate the 37 other mysterious deaths, but it may be very challenging to prove Poggiali had any involvement. Daniela Poggiali lives in the town of Lugo.

Gawker reported that Poggiali’s coworkers were suspicious that she might have had something to do with the death of the 38 victims because the condition of certain patients would worsen during her shift, often leading to their death.

“We wondered whether these deaths could be so frequent without anyone doing anything,” said a coworker.

Others who worked with the nurse described her as a “cold, polished” person who would use vengeful tactics against patients she disliked. For example, Poggiali would sometimes sedate patients that were incessantly bothering her during her shift or even give patients laxatives to force nurses working after her to deal with the consequences. Another coworker described the nurse as a “cold person but always eager to work.”

The local Corrierenewspaper reported on the nurse’s shocking murders, stating that the nurse even took selfies with her camera phone minutes after her victims had passed away. Police searched Poggiali’s phone and found a photo taken several months ago featuring the nurse giving a thumbs up in front of a patient she had just killed.

“I can assure you in that all my professional years of seeing shocking photos, there were few such as these,” said prosecutor Alessandro Mancini.

For more on shocking murders, read about the mother who killed her three disabled children.