Kuboki told police the attacks were timed for the patients to die when she was not at work so she would not have to deal with heartbroken relatives.
The nurses worked in pairs, Asahi Shimbun reported, and Kuboki would administer the deadly chemicals while her colleagues were busy in briefings as shifts changed over.
The 31-year-old, who worked at Oguchi Hospital in Yokohama, was busted Saturday on suspicion of murdering 88-year-old Sozo Nishikawa in September 2016.
"It would be troublesome if that responsibility fell on me", Kuboki said.
While in custody, Kuboki admitted to police that she killed the two men - and "about 20 other patients" - over the course of two months in 2016 so she wouldn't have to explain the circumstances of their deaths to their families.
Bubbles are an indication that a bag has potentially been tampered with.
The same chemical has been found in the bodies of two other patients who died aged 78 and 89, the news agency said. Police launched an investigation after discovering traces of benzalkonium chloride in the nurses' station, the same chemical found in Yamaki and Nishikawa.
Police began investigating following Nishikawa and Yamaki's deaths and found 10 unused antiseptic-laced IV bags with tiny holes in the rubber seal - indicating they were punctured with a syringe.
It is believed the suspect has not worked as a nurse since the 2016 death.
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.
But police are unconvinced that Kuboki had only "helped" gravely ill patients to die - there's evidence suggesting that healthier patients were also poisoned.
After her arrest, Kuboki allegedly confessed to killing up to 20 patients, but claimed she targeted only the very sick. Forty-eight deaths occurred between July and September 2016, including five on a single day in August and four on one day in September, according to the Japan Times. At first, Kuboki denied all involvement, even expressing her sympathy for the families of the deceased and saying she noticed nothing suspicious around the hospital.
Kuboki's former co-workers said they were shocked to hear about the nurse's arrest. She has not yet entered a plea.
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