The dog that killed a 3-day-old Mira Mesa boy has been euthanized at the owner's request, County of San Diego Department of Animal Services said Wednesday.

"The owners of the dog “Polo” came to the Department of Animal Services this morning and relinquished the dog for euthanasia," said Dan DeSousa, the deputy director of the department, in a statement. "The dog was euthanized at the owner’s request and this closes the Department’s role in this incident."

Polo was with his owners and their newborn, Sebastian Caban, in a bed on April 21 when the baby's mother suddenly coughed, according to an autopsy report.

The dog bit the baby in the head in what the San Diego County Medical Examiner’s Office has called an accident.

Polo was under quarantine and in the custody of the Department of Animal Services until the family made the decision.

On Tuesady, the San Diego Humane Society sent a letter to its volunteers confirming that Polo had been adopted by the owners six months ago.

The agency explained that there was no sign of trouble with the 2-year-old American Staffordshire Terrier.

“There was not a fragment of aggression seen in this dog before tragedy struck,” said Gary Weitzman, DVM, MPH, CAWA, President and CEO of the San Diego Humane Society.

"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," Weitzman said.

"His records tell of a happy, social dog who enjoyed going on walks and playing with other dogs," he added.

He added that the agency sends its thoughts to the family of the baby.

"What happened was tragic and I can’t even begin to fathom the pain this family is suffering," Weitzman said.

Shortly after the incident, the couple made two unsuccessful 911 calls, SDPD officials confirmed.

They ended up driving the newborn to Rady Children’s Hospital where medical staff pronounced him dead.