ROME—There’s nasty. And then there’s diabolic. Daniela Poggiali, an Italian nurse who was just arrested for the suspected death of at least one patient and as many as 38 more in the north of Italy, would appear to be both.

When Poggiali, 42, was feeling nasty, according to her colleagues, she would ply her elderly patients with laxatives at the end of her shift and leave the “dirty work” for the nurses taking over for her at the Umberto I Hospital of Lugo near Ravenna. When she was feeling diabolic, according to state prosecutor Alessandro Mancini, she would ply the patients’ veins with potassium to kill them.

In one case last April, the narcissistic nurse then took smiling selfies with the open-mouthed cadaver of 78-year-old Rosa Calderoni. In one cellphone photo, a cropped version of which was published in the Italian press, she is seen imitating the dead woman by opening her own mouth in a sly smile. She had superimposed a caption mocking her alleged victim: “Brr,’ mmh, la vita e la morte, mmh” (life and death).

“If someone is capable of taking a photo like that, they are capable of doing a lot more,” Mancini told reporters outside her arraignment hearing.

Poggiali was often the only nurse on the night shift at the regional hospital that served mostly elderly patients in the absence of a nursing home. Most of the 38 alleged victims were gravely ill or suffered terminal conditions. According to Poggiali’s colleagues, she often complained about the whiney patients and pushy relatives and often appeared to be acting suspiciously. According to a police investigation leaked to Corriere della Sera newspaper, a nurse at the hospital told investigators that when patients were particularly cranky, she often said, “Don’t worry, I’ll take care of it.”

When Calderoni’s family filed a complaint and request for an autopsy, authorities found extraordinary high levels of potassium in the elderly woman’s blood. It takes around 48 hours for a patient poisoned with potassium to die, which gave Poggiali plenty of distance from the deaths. Two other people died on the same day as Calderoni. The week before Calderoni died, a relative of the hospital director, who Poggiali reportedly did not get along with, also died.

In fact, during the first four months of 2014, 39 people, including Calderoni, died and the hospital staff started to get suspicious. According to the court documents, the hospital administrator told the supervisors after Calderoni’s death that if anyone else dies, not to call the morgue, call the cops. Police then spent the next five months investigating Poggiali, careful not to leave her alone in the hospital ward. No one else died.

At least two of Poggiali’s colleagues have given evidence against the nurse. According to court documents published in the Italian press, Marinella Felloni told investigators that she saw Poggiali stuff around €300 worth of medicine into her backpack at the end of her shift one day last spring. When she asked Poggiali about the drugs she appeared to be stealing, Poggiali is supposed to have answered, “I pay my taxes.” Felloni then told her superiors about the backpack, but when they checked inside, they found only a bouquet of cyclamen flowers tied with a black ribbon.

Another nurse, Sara Pausini, told investigators that Poggiali often argued with colleagues and treated patients not assigned to her. “She was tireless and often seemed in a state of euphoria,” Pausini told police, according to the documents.

Questions are being raised why no one caught Poggiali when people started dying last spring. But because most of the patients were elderly and unwell, autopsies were never carried out and the patients were interred without suspicion until Calderoni’s family demanded answers. Poggiali, who was laid off before she was arrested, has so far refused to answer questions put forth by police, according to the preliminary judge who signed her arrest warrant, Rossella Materia, who described her actions as psychotic. “The defendant derives pleasure from the humiliating death of those who perish in such a weak state,” Materia wrote in her court document. “It becomes a form of self affirmation.”

Poggiali’s defense lawyer, Stefano Dalla Valle, has refused to comment on his client’s case. Poggiali will face a preliminary hearing later this year.