A 49-year-old nursing home worker in Ontario 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.
The victims were residents in two long-term care facilities where Ms Wettlaufer worked and were between 75 and 96 years old.
Police said seven of the victims were given a fatal dose of a drug.
The victims were five women and three men and were all residents of Caressant Care in Woodstock and Meadow Park in London, Ontario.
"The victims were administered a drug... there are obviously a number of drugs that are stored and are available in long-term care facilities," Woodstock Police Chief William Renton said at a news conference.
The chief declined to comment on a possible motive, but did add that investigators are confident that they have identified all victims.
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.
Woodstock is a car industry and agricultural town of some 37,000 people, about 128km (79 miles) west of Toronto.
In the residential neighbourhood where the Caressant Care facility is located, few people were seen coming and going.
Media were staked outside the property, across from a local school.
In the hallway of the adjoining retirement residence, a television could be heard playing the news of Ms Wettlaufer's arrest.
An employee at the facility, who was not authorised to speak on the record, told a reporter that it was a shame the actions of one person could colour a whole profession.
Ms Wettlaufer was also an employee at Christian Horizons, the long-term care facility confirmed.
A spokesperson for the organisation, which was not named in the police investigation, said she stopped working for them in 2007.
"Christian Horizons fully intends to cooperate with the authorities in their investigation in this matter," said chief executive Janet Noel-Annable in a statement.