TOKYO - A 31-year-old former nurse was arrested for murder on Saturday (July 7), two years after a series of "angel of death" killings occurred at a hospital in Yokohama.

Ayumi Kuboki has reportedly told police that she injected a chemical, used in disinfectants, into the intravenous drips of at least 20 of her patients. Another 10 unused IV bags had also been found with puncture marks.

The deaths occurred between July and September 2016, when Kuboki was working at the 85-bed Oguchi Hospital for the elderly, now renamed the Yokohama Hajime Hospital.

"I hated seeing the rapid deterioration in their condition," Kuboki was quoted by TBS News as telling investigators. "I did not want them to die during my shift. It was troublesome and difficult having to explain to their family members."

She was nabbed after traces of the chemical were found on her nurse uniform, and has confessed to the crimes during voluntary questioning by the police.

A total of 48 people died at the hospital during the three-month period, including five patients in one day in August 2016, and another four on a single day a month later.

Police have confirmed that at least four of them have died under suspicious circumstances, though it is said that it would be difficult to ascertain the precise cause of death for many of the victims as their remains had been cremated.

Hospital officials had not suspected that anything was amiss at first, owing to the old age of the patients and their debilitating health condition when they were admitted.

But the alarm was raised when a nurse noticed the abnormal presence of small bubbles in the IV drip bag used by Mr Sozo Nishikawa, 88, on Sept 18, 2016.

Two days later, Mr Nobuo Yamaki, 88, was also found poisoned to death in the same ward. Their deaths, ironically, bookended the Respect For The Aged Day public holiday.

Police have long suspected an insider was behind the so-called "mercy killings", given that the culprit seemed to have medical knowledge and connections to the facility.

Kuboki had, in interviews with various domestic media outlets in 2016, protested her innocence and insisted she was not involved in the "heinous" crime.

"I feel very sorry for the victims as well as their families," she had told Kyodo News.

She also told Mainichi Shimbun, when asked about speculation that it was a nurse was behind the series of deaths: "I shudder to think there are such people among my colleagues. Actually, some have accused me of being the criminal, and I was extremely hurt and shocked by the level of distrust."

Family members of Mr Nishikawa and Mr Yamaki told public broadcaster NHK that they are relieved an arrest has been made.

Even so, Mrs Nishikawa said: "There was every chance that my husband might have died because he was sick. But the fact that he was murdered only means that it was my own poor judgment in choosing the hospital."

Mr Yamaki's eldest son, meanwhile, was quoted as saying: "I want to know why a person, who found herself fit to work as a nurse, could commit such a crime."