Austin Harrouff, a Florida State University student, is accused of murdering married couple John and Michelle Stevens as they sat in their driveway of their Tequesta, Florida home. The 19-year-old reportedly stabbed the couple multiple times and then bit the man’s face.

In his only televised sit-down interview, Harrouff’s father, Dr. Wade Harrouff, a prominent dentist in the community, reveals to Dr. Phil his account of what led to the incident. Dr. Harrouff shares what he called his son’s “manic behavior” at dinner the night of the murders, and his claim that his son underwent a dramatic personality change two weeks prior to that fateful night.

“It’s the worst thing I could ever think of. This is the biggest nightmare I could ever even dream of,” an emotional Harouff tells Dr. Phil in the episode airing Wednesday, September 7. “I’m deeply sorry for what my son did to those people. And I’ll apologize for him, because my own son would have never done that ... He’s such a good person. He would never knowingly do something like that. I’m truly sorry to those people.”

Harrouff, who also allegedly stabbed the Stevens’ neighbor who was attempting to help the couple, has reportedly tested negative for common drugs, but some speculate he may have been under the influence of the hallucinogenic drug ‘Flakka’, also known as “gravel” or “the zombie drug.”

Dr. Phil explores the circumstances surrounding the gruesome double murder in a ripped-from-the-headlines all-new episode. Check here to see where you can watch.