Pamela Porillo left the two dogs alone for a few hours in her Lackawanna home to run an errand.
When she returned, she found Sierra had killed Maggie. And then she learned that Sierra was likely part pit bull, the breed that has picked up a reputation for violence because of those who breed them for fighting.
Animal experts said Maggie's killing is a cautionary tale about taking in dogs whose background is unknown or unclear.
"If I would have known it was a pit bull, I wouldn't have allowed it in my house and near my dog," said Porillo, who got Sierra after her other schnauzer, Daisy, had died. "I just wanted my little Maggie to have a playmate."
Porillo blames Buffalo Pug and Small Breed Rescue for giving her a dog that turned out to be part pit bull.
Officials with the Alden-based organization said the dog's breed is still not clear, and that Porillo shares the blame for leaving the dogs together and unsupervised in a confined area.
"It's anybody's best guess when you are trying to guess the mix," said Chad Salvatore, founder of the nonprofit organization that gave Sierra to Porillo. "After the incident, we had (Sierra) here for a week; she doesn't have any aggression at all."
Local animal officials - including the head of the SPCA - identified her as a pit bull.
Frederick S. Grasso, Lackawanna animal control officer, responded to the incident.
"The dog, in fact, was a pit bull," he said in a police report. "The shape of the skull and the muscular front shoulders were a dead giveaway to breeding."
Salvatore said Sierra was a stray and acquired from a Kentucky Humane Society. He said the dog was examined by a Syracuse veterinarian who could not determine its breed.
A volunteer from Buffalo Pug and Small Breed Rescue who dropped off the dog in March said it was a Jack Russell terrier mix, Porillo said. Porillo said she had responded to one of the organization's advertisements to become a foster parent.
Carlise Schweitzer, the volunteer who delivered Sierra to Porillo's home, said Porillo told her that her schnauzer Maggie had "playfully attacked" Sierra. Schweitzer said because of that incident, it was irresponsible of Porillo to confine the two dogs and leave them alone.
But Porillo said 21-pound Sierra was too energetic and larger than she requested, and after a week she didn't want her. The organization said it would take a week to place Sierra in a new foster home, Porillo said, and she decided to keep her until then.
In late March, she said, she left the two dogs in a "fenced-in" area in a bedroom while she ran an errand. She returned to find her 12-pound schnauzer dead.
"There was little Maggie loaded with blood all over the place," she said. "Can you imagine my horror? I couldn't even cry."
Barbara Carr, executive director of the Erie County Society for the Prevention of Cruelty to Animals, saw a picture of the dog and said its physical traits are consistent with a pit bull - from its ear set to its color.
"It certainly appeared to be a pit bull," Carr said. Carr said animal rescue agencies have good intentions but often lack the basic knowledge of identifying breeds, which can have deadly consequences.
"They are well-meaning people, but look at the result when you don't know what you are doing," she said. "Anybody in the animal rescue field has a responsibility to know what they are doing and what breeds they are dealing with."
Carr said dog owners also have a responsibility.
"Anybody taking a dog into their home has a responsibility to find out what is this animal," she said, "and what can I expect out of it."