A 49-year-old nursing home worker in Ontario, Canada has been charged with eight counts of first-degree murder spanning several years.
Elizabeth Tracey Mae Wettlaufer of Woodstock, Ontario, appeared in court on Tuesday morning (local time).
The victims were residents in two long-term care facilities, Caressant Care in Woodstock and Meadow Park in London, Ontario, where Ms Wettlaufer worked.
Police said seven of the victims, aged between 75 and 96 years old, were given a fatal dose of a drug.
Caressant spokesman Lee Griffi said the accused was a registered nurse and left their employment approximately two and a half years ago.
"We deeply regret the additional grief and stress this is imposing on the families involved," Mr Griffi said in a written statement.
The deaths took place between 2007 and 2014.
Police revealed little details about the motive or circumstances surrounding the deaths in a press conference held on Tuesday morning, but said that the investigation into all eight deaths started on 29 September after police in Woodstock received certain information.
"The information came to our attention, and unfortunately the manner in which it came to our attention will form part of the evidence and we can't speak to that," Woodstock Police Chief Bill Renton said.
Police said they do not know if there were other victims, but that the nurse worked in other facilities.
They have urged the public to come forward if they have any information.