Photo/IllutrationA nurse who is suspected of killing a patient by poisoning in 2016 heads to a prosecutors' office in a police car in Yokohama’s Naka Ward on July 9. (Shinichi Iizuka)

YOKOHAMA--A former nurse suspected of slaying an elderly patient by poisoning at a hospital here said she may have killed around 20 other patients, according to investigative sources.

“I also administered antiseptic solution with other patients,” Ayumi Kuboki was quoted as saying. “I did that to about 20 patients.”

Kanagawa prefectural police referred the case to prosecutors on the morning of July 9.

Kuboki, 31, was arrested on July 7 on suspicion of killing Sozo Nishikawa, 88, on Sept. 18, 2016, by deliberately administering him with a quantity of dense antiseptic solution in a short period of time at Oguchi Hospital in Yokohama’s Kanagawa Ward.

Police believe she administered the agent between 3 p.m. and 4:55 p.m. that day, according to the sources. Nishikawa was confirmed dead at 7 p.m.

Kuboki told investigators that she poisoned more patients, including Nobuo Yamaki, 88, who shared a room with Nishikawa at the hospital, and who died two days after Nishikawa.

She confessed during voluntary questioning, and police are investigating how she did it, according to the sources.

As for her motive, Kuboki told police that she wanted to ensure the patients died when she was not at work to avoid situations in which she would have to explain the circumstances of their deaths to bereaved families.

Nurses at Oguchi Hospital are expected to break the news to families if their patients die while they are on duty.

“It would be troublesome if that responsibility fell on me,” Kuboki was quoted by the source.

An autopsy conducted on Sept. 21, 2016, detected a surface-activating agent component found in benzalkonium chloride, a solution with strong antiseptic properties, in Yamaki’s body and his intravenous drip bag.

The autopsy was conducted as another nurse at the hospital noticed bubbles in Yamaki's intravenous drip bag when he died.

Police determined that Yamaki’s death was due to poisoning.

After the discovery, police opened an investigation into the death of Nishikawa, and determined he also died of poisoning. Doctors had initially attributed the cause of his death to disease.

Benzalkonium chloride is used to disinfect the hands and wounds, and is easily available throughout the hospital.

Police also discovered the surface-activating agent component in the bodies of two other patients--an 89-year-old man and a 78-year-old woman--who died around that same time as Nishikawa and Yamaki.

Kuboki, a resident of Yokohama’s Tsurumi Ward, gained a nurse’s license in 2008 after attending a nurse training school following her graduation from high school in the prefecture.

She began working at Oguchi Hospital in May 2015 after working for a separate hospital.

“We had had no awareness that she was a problem employee,” a worker at the hospital said.

An employee at another hospital said, “She was the kind of person who was hard to figure out what she was really thinking, but she was considered competent.”

Nishikawa and Yamaki were hospitalized in a room on the fourth floor.

A series of troubling incidents were reported on that floor between April and August 2016, where nurses’ uniforms were found ripped and drinks were found spiked with a foreign substance.

After the deaths of Nishikawa and Yamaki, police suspected that nurses stationed on the fourth floor, including Kuboki, may have known something about their deaths.

But police could not obtain visual evidence as no surveillance cameras were installed within the hospital at that time.

Police also discovered unused intravenous drip bags at the nurses’ station tainted with the antiseptic solution.

They also found tiny punctures in about rubber seals of the 10 drips, indicating the agent had been injected with a syringe, according to the sources.

When police investigators checked all uniforms worn by nurses in charge of the patients on the fourth floor, they detected the surface-activating agent component only in Kuboki’s uniform. It was found around a pocket.

Police questioned Kuboki in late June, after she quit the hospital. She admitted to the slaying allegations on the second day of questioning, according to the sources.

In an interview with The Asahi Shimbun in December, Kuboki said, “I have worked hard to help patients die in peace, so I was shocked by the incidents.”

(This article was written by Hirohisa Yamashita, Kazuya Ito and Naoto Iizuka.)