When Killian, the Jordan family’s normally friendly dog, started to growl and snarl at 7-month-old Finn’s babysitter, the parents knew something must be wrong.

Benjamin and Hope Jordan had hired Alexis Khan, 22, to care for their son shortly after they moved to Charleston, S.C. Although the sitter passed a background check and the Jordans trusted her, their loyal dog did not.

“We noticed the dog was getting very defensive when Alexis was around. He would growl and stand between her and our son. His hair would stand up on the back of his neck and we knew something was up,” Jordan told ABC News.

More ABC US news | ABC Health News The suspicious parents decided to place an iPhone under the couch to record what was happening in the house while they were at work. The horrifying audio revealed that Khan, who’d been taking care of Finn for five months, was cursing at and slapping the baby when he cried, according to court documents.

“We just could not believe it,” said Jordan.

The distraught parents brought Finn to the hospital to be examined, and thankfully he was fine. The doctors alerted the police, who interviewed Khan and got a confession.

Khan appeared in a Charleston court in September and pleaded guilty to assault and battery charges. The conviction came about a year after the Jordans first recorded her. She was sentenced to one to three years in prison, and her name will be placed on a child abusive registry, preventing her from working with children again.

The Jordans are very grateful to their hero, Killian. Now, they’re hoping the Lab and German Shepherd mix will be able to help others, too. He’s in training to become a psychiatric services animal.

More on Vetstreet.com:

* 7 Things Pet Owners Do That Drive Veterinarians Crazy

* Pit Bull Named Hero Dog of 2013

* 10 People-Pleasing Dog Breeds