A nurse suspected of killing at least 20 elderly patients in a hospital in Japan has told investigators that she added disinfectant to their intravenous drips shortly before finishing her shifts because it was “a nuisance” to explain to relatives when a person had died during her working hours.

Ayumi Kuboki, 31, has been arrested on suspicion of killing 88-year-old Sozo Nishikawa at Oguchi Hospital, in the city of Yokohama, south-west of Tokyo, in September 2016. She is also being questioned over the death of Nobuo Yamaki, also 88, two days later.

Since her arrest, Kuboki has admitted to police that she killed the two men as well as “about 20 other patients” in her care, the Asahi newspaper reported. Police believe she deliberately contaminated the patients’ intravenous drips with a disinfectant containing toxic benzalkonium chloride, killing the first two victims in a matter of hours. Kuboki has reportedly told police that she wanted to make sure that the patients died when she was not on duty so she would not have to explain the circumstances of their deaths to their families.

“It would be a nuisance if that responsibility fell on me,” she said.

Doctors had initially believed Mr Nishikawa and Mr Yamaki had died of natural causes, although autopsies discovered the disinfectant in their bodies. 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.