A 42-year-old Italian nurse in the northern town of Lugo was arrested over the weekend on charges that she killed some 38 patients under her care because she found them — or their families — “annoying.”

According to the Daily Mail, Daniela Poggiali had a coworker take a photo of her next to a recently deceased patient who authorities now believe that she murdered with a lethal injection of potassium chloride.

Suspicions arose after the April death of 78-year-old patient Rosa Calderoni, whose autopsy revealed dangerously high levels of potassium in her bloodstream. High doses of potassium can stop the heart. The investigation came to an inconclusive end, however.

Prosecutors said that the case hit “insurmountable difficulties” due to the fact that potassium fades from the bloodstream within a matter of days.

Coworkers describe Poggiali as “cold” and reported that she joked about killing patients with potassium in the past. She was also known for giving her patients high doses of powerful laxatives at the end of her shifts so as to “embarrass” the nurses coming on duty after her.

Lead prosecutor Alessandro Mancini was quoted in the newspaper Il Resto Del Carlino as saying, “There are the other 38 cases of suspicious deaths and of these, ten are very suspicious.”

“Investigations are continuing,” he said, “but there are insurmountable difficulties.”

Poggiali was reportedly “unperturbed” as she was taken into custody and charged with murder.