Two elderly residents of a Newcastle nursing home have died from the same illness caused by low blood sugar.
A third resident was also admitted to hospital with the same hypoglycaemia symptoms but survived.
A doctor reportedly raised the alarm when he discovered all three were residents of the Sugarloaf Gardens Nursing Home in Wallsend and were not dependent on insulin, which was allegedly injected into them.
The homicide squad is working with Newcastle police who are investigating whether there is any criminal involvement in the deaths.
An 83-year-old woman was admitted to John Hunter Hospital in Newcastle on October 18 with hypoglycaemia symptoms and died the next day.
On October 19, a 91-year-old woman and a 81-year-old man were also admitted to hospital suffering from hypoglycaemia.
The man died 10 days later while the woman survived and was released from hospital.
Diane Dean, a visitor to the home on Wednesday, said a lot of people would be shocked.
"There will be a lot of people here who are quite shocked and probably want to remove their residents from the facility," she told the Seven Network.
Parent company Summit Care issued a statement saying it was determined to continue looking after residents' wellbeing.
Newcastle City Police Commander Superintendent John Gralton said no stone would be left unturned in the investigation.
"We are working with management of the aged-care facility and the involved families as the investigation continues," he said.
"We are still awaiting the formal evaluation of medical evidence to determine the circumstances leading up to the deaths and any involvement of other parties.
"The presence of any criminal intent is still being determined.
"The evaluation of medical evidence will assist in this regard."Supt Gralton appealed to families of residents and said they should contact police if they had suspicions a criminal act had been committed.