A 31-year-old former nurse was arrested Saturday for allegedly murdering an elderly patient in 2016 at a Yokohama hospital where at least three other patients also died under suspicious circumstances, police said.

Ayumi Kuboki has admitted to killing Sozo Nishikawa, 88, who died of surfactant compound poisoning on Sept. 18, 2016 at what was then Oguchi Hospital where she worked, the police said. Kuboki told them she injected disinfectant into intravenous drip bags used to treat about 20 patients, according to investigative sources.

(Oguchi Hospital in March 2017)

Traces of a surfactant compound were found in the bodies of three other patients who died. One of them, Nobuo Yamaki, 88, died two days after and in the same room as Nishikawa, and the same type of toxic substance was found in his body and in an IV drip bag used to treat him, according to police.

Kuboki told police she repeatedly injected disinfectant into drip bags, the sources said, adding police are investigating the possibility the nurse killed Yamaki as well.

Between July and Sept. 20, 2016, 48 patients died at the hospital, including five patients on a single day in late August and four on one day in early September.

The substances found in the bodies of Nishikawa and Yamaki were the same type as the disinfectant used at the nurses' station on the fourth floor of the hospital, where their room had been.

Among about 50 unused drip bags at the nurses' station, about 10 were found with small holes. The police believe someone may have injected the disinfectant with a syringe.

Soon after the deaths of Nishikawa and Yamaki, the police began investigating the case as possible murders committed by someone with knowledge of the hospital and medicine.

Shortly after police began investigating the deaths, the nurse told Kyodo News that she was not involved in the incidents and had not noticed anything unusual at the hospital.

While flatly denying that she laced medical drips with poisonous substances, the nurse said she was "shocked" by the incidents and "felt very sorry for the victims as well as their families."

Beginning about half a year before the death of Nishikawa, a series of suspicious incidents occurred at the hospital in Yokohama's Kanagawa Ward. In April shredded nurses' aprons were found, in June part of a patient's medical records was lost, and in August a nurse tasted what she believes was bleach in a bottled drink.

According to her mother, Kuboki told her she was afraid of working at the hospital at night and wanted to quit her job.