A nurse working in a northeastern Italian hospital has been arrested as part of an investigation into the deaths of 38 patients who, cops said, she may have injected with potassium chloride because they or their family members were "annoying,"

Coworkers suspected Daniela Poggiali, 42, because patients would suddenly worsen and die during her shift. They described her as a "cold, polished" person who sometimes sedated patients who bothered her during her shift. They also suspected she gave patients laxatives to embarrass the colleagues who worked after her.

"We wondered whether these deaths could be so frequent without anyone doing anything," a coworker told Corriere della Sera.

The investigation and arrest resulted from the death of a 78-year-old patient who was found to have died from a potassium chloride injection, the New York Post reported. But homicide may be more difficult to prove in the case of the earlier victims.

Prosecutors told Corriere that potassium leaves no trace after a few days, making it challenging to investigate.

Police searched Poggiali's phone and found a photo of the nurse giving a thumbs up next to the body of a patient moments after that patient's death, Corriere reported.

[H/T NY Post, Photo: Twitter]