The owners of the dog that mauled their newborn baby boy to death relinquished the dog for euthanasia on Wednesday.
The mauling of the baby is weighing heavily on the group that rescued the dog involved.
The president of the San Diego Humane Society sent a letter to staff and volunteers to address the tragedy, which left a 3-day-old baby dead in Mira Mesa last week.
Humane Society officials said the dog, Polo, underwent temperament testing -- a test all dogs at the shelter go through.
The test is designed to assess the personality of the pet to determine if the pet is good with children or cats and to make sure whoever wants to adopt them are the right ones.
Through the testing, officials said Polo was found to be a family friendly dog, cheerful and full of energy.
News of the mauling stunned Humane Society staff, who said it is out of character.
In a letter to members, staff and volunteers, Humane Society President and CEO Dr. Gary Weitzman said, in part:
"The first question we asked was whether we had seen any signs of aggression in this dog before adopting him out. That answer is no. His records tell of a happy, social dog who enjoyed going on walks and playing with other dogs. He was adopted quickly and greatly loved by his new family.
This horrible accident was one that all of us in animal sheltering dread and not a single one of us could have foreseen. There was not a fragment of aggression seen in this dog before tragedy struck. But tragedy did strike and there was nothing any of us could have done to prevent it. Our hearts go out to this family."
Officials said Polo was adopted about six months ago and underwent more temperament testing.
Police said the newborn was in bed with the parents and the dog when the child's mother coughed. The dog was apparently startled by the cough and suddenly attacked the baby, police said.
The newborn was pronounced dead at the hospital.
The parents gave permission Wednesday for the dog to be euthanized and the dog was put down.