Nearly 50 people died in a Yokohama hospital for the elderly between July and last month, in what Japanese police increasingly suspect to be a series of mercy killings.
The perpetrator behind the deaths at the 85-bed Oguchi Hospital in Japan's second-largest city is likely to be an insider with medical knowledge and who has connections with the facility, police said.
No arrests have yet been made in the probe, which was launched last month after two patients, both aged 88, were poisoned to death within days of each other.
The modus operandi of the culprit, who has been dubbed the "angel of death", has been to inject a chemical used in disinfectants into the patients' intravenous drips.
The police are trying to determine the cause of death of 46 patients who had died earlier, but they admit this may be difficult as many of the bodies have been cremated.
"We see many people die as we take in elderly patients, but we had the impression that the number of those dying was increasing a bit," a hospital official was reported as saying.
Another official added that the hospital initially thought the spike in deaths was because there were more seriously ill patients.
Five patients died on a single day in late August and another four on a single day early last month.
But hospital officials did not suspect anything amiss until Mr Nobuo Yamaki died on Sept 20 and a nurse noticed the abnormal presence of small bubbles in the IV drip bag he was using. He died two days after Mr Sozo Nishikawa, who was found to have been poisoned.
The two deaths occurred around the time of a three-day long weekend last month to mark the Respect For The Aged public holiday, with the hospital thinly staffed.
Police suspect the killer might have targeted the victims indiscriminately. Ten of the 50 unused drip bags stored behind the nurses' desk on the fourth floor of the hospital were found to have puncture marks.
Many of those warded at Oguchi Hospital, which is not taking in any new patients while the probe is ongoing, are bedridden and have trouble eating on their own.
This is not the first incident to mar the hospital in recent months. In April, a nurse's apron was found slashed, while in August, an employee suffered blistered lips after drinking a beverage purportedly laced with bleach.
The latest case, if proven to be a mercy killing, would be similar to that of Danish nurse Christina Hansen, who was given a life sentence in June for murdering three elderly patients and trying to kill a fourth via drug overdoses.
In 2006, German male nurse Stephan Letter was convicted of administering lethal injections to 28 mostly elderly patients.
An editorial in the English- language Japan Times last Friday called for security at care facilities to be beefed up.
"Security systems in hospitals and care facilities are often said to be based on a positive view of human nature," it said.
"But, in reality, many incidents have taken place where vulnerable patients have been exposed to harm - and, in some cases, lost their lives - in these supposedly safe places."
In 2014, a former care worker at a nursing home in Kawasaki was said to have killed three elderly residents by pushing them off a balcony.
In July this year, 19 inmates at a home for the disabled in Sagamihara were slashed to death by a former employee.