43-year-old Anna Castagna of Brooklawn, N.J. is charged with murdering 88-year-old Mary Groatman.
Prosecutors say Castagna killed her by getting her to ingest antifreeze.
"It really is a heart-wrenching act," said Gloucester County Prosecutor Sean Dalton.
At a Thursday afternoon news conference, Dalton announced that Castagna was being charged with murdering her husband's grandmother, a woman the couple lived with and Castagna took care of for many years.
Recently, 88-year-old Mary Groatman suffered injuries in a fall. She was staying at the Deptford Center for Rehabilitation and Healthcare.
Investigators say on Sunday afternoon, Castagna went to visit the elderly woman and got her to ingest antifreeze. Later, staff members discovered Groatman unconscious, and she was rushed to Underwood-Memorial Hospital.
"It was determined by the ER staff at Underwood Hospital that she had ethylene glycol, the main ingredient in antifreeze, in her system," said Dalton.
"The investigation further determined that the accused had, in the time period leading up to this incident, researched the effects of ingesting antifreeze on a human body," he added.
Police also have evidence that Castagna purchased the antifreeze on Sunday morning before visiting her husband's grandmother.
Prosecutors say prior to the recent fall and this antifreeze incident, Groatman was in good health for her age.
"To have her life taken as a result of this really is a sick act on the part of the accused," Dalton said.
Groatman leaves behind children, grandchildren and great-grandchildren. Prosecutors would not reveal a motive and said the investigation is ongoing.
Officials at the rehab center told Action News they are cooperating with the New Jersey Department of Health and have no comment.
Castagna is being held at the Gloucester County Jail in Woodbury on $250,000 cash bail.