Ayumi Kuboki allegedly poisoned the patients by lacing their intravenous drips with toxic antiseptic chemicals at specific times, Japanese newspaper the Asahi Shimbun reported Tuesday.
"Explaining to family members about deaths was cumbersome if a patient died during my working hours", she told investigators, Jiji said.
A Japanese nurse who may have killed dozens of elderly patients wanted to ensure that her terminally ill victims died anytime but on her watch, due to the "nuisance" of facing relatives with tragic news, Japanese media reported.
Police were already investigating the deaths of two patients, including the 88-year-old. "It was troublesome and hard having to explain to their family members". The men, who'd shared a hospital room, died within two days of each other.
"It would be a nuisance if that responsibility fell on me", she explained.
Police believe she deliberately contaminated the patients' intravenous drips with a disinfectant containing benzalkonium chloride, killing the first two victims in a matter of hours.
Kuboki quit the hospital before she was questioned over the deaths in June, before her arrest on July 7.
Subsequent investigations also found the cleaning agent in the bodies of an 89-year-old man and a woman aged 78 who had died around the same time.
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Staffers told Asahi Shimbun there were no red flags. They declined to confirm the broader investigation.
Another said: "She was the kind of person who was hard to figure out ... but she was considered competent".
But between April and August 2016, the fourth floor was plagued by a series of unsettling problems - including ripped nurses' uniforms and drinks that were found spiked with unknown substances.
When police investigators checked all uniforms worn by nurses in charge of the patients on the fourth floor, they found the antiseptic on only one uniform - that worn by Kuboki - around the pocket. Police zeroed in on Kuboki after analyzing hospital worker uniforms, finding that only hers carried traces of the chemical.
The Asahi Shimbun also reported she had admitted injecting the drips of around 20 patients with disinfectant.
Former colleagues said they were shocked to learn that Kuboki, who started working at the hospital in 2015, had been arrested.
Kuboki has admitted to killing "at least 20 elderly patients" in her time at Oguchi.
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