The father of "cannibal killer" Austin Harrouff has broken down as he apologised to the families of his victims and revealed what he downed before the attack.

Dr Wade Harrouff said his life had become "the biggest nightmare" after his 19-year-old son stabbed a random couple to death before trying to eat the man's face off.

He also revealed that his son's mental health had noticeably declined in the two weeks leading up to the attack.

He said on the night in question his son had down vegetable oil before storming out, complaining about the slow service.

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It was on his way home from the restaurant that he discovered Michelle Mishcon, 53, and her husband John Stevens, 59, sitting in their garage watching TV.

He stabbed both of them to death. It took multiple officers, a police dog and a Taser to pull him off Stevens' dead body, which he reportedly was trying to eat.

Austin Harrouff leaving Duffy's Sports Grill before stabbing a man and wife to death. Photo / AP
Austin Harrouff leaving Duffy's Sports Grill before stabbing a man and wife to death. Photo / AP

"I'm deeply sorry for what my son did to those people," he told Dr Phil. "And I'll apologise 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," he said.

Police believe Harrouff was taking the synthetic drug Flakka, which can make the user think they have herculean strength.

However some from Florida State University, where Harroff was studying exercise science, said they believe he was taking steroids and had bulked up very quickly in the past year.

His mum Mina called 911 after he stormed out of the restaurant, saying she was concerned about his welfare.

"It seems like he's a little delusional," she added. "There's this recent change that we're noticing. He was out to dinner with his dad today and he took off.

"He says he feels immortal and like a superhero. I don't know what's going on with him. The only thing he has on is a switchblade type pocket knife," she said.

Police said while he was being taken to hospital he told them "you won't find any drugs in my system."

He tested negative for drugs like cocaine but the Flakka tests take longer.

In the month leading up to the attack on the Stevenses, Harrouff posted 41 YouTube videos, progressively getting more and more bizarre.

"I've got a psycho side and a normal side. I've lost my mind. Help me find it," he said in one video.