Police in northeast China's Heilongjiang Province have arrested a suspect but have not revealed his identity or motive.
The victims identified the perpetrator as a worker at the nursing home, while a staff member there said the man, in his 30s, was also a patient for mental illness, state-run Xinhua news agency reported.
Meanwhile, an investigation was launched against Wei Mingfu, head of the Zhenxiang Township government in Qinggang County, where the incident happened, Liang Xun, deputy head of the county's civil affairs bureau, and Su Zhanhai, head of the Zhenxiang Township nursing home, for negligence.
The unidentified perpetrator tied up four patients aged between 53 and 80 years and removed their private parts with a dull razor at the nursing home on Tuesday afternoon.
Two of the victims were bedridden in the institution, while the third is mentally challenged.
The fourth victim was not found until yesterday when police carried out a thorough inspection of the nursing home, the report said.