The story just keeps getting more and more horrifying. Chris Watts has already plead guilty to murdering his wife and two daughters, but as more of the story comes out, it just gets worse. In a new interview with the Colorado Bureau of Investigation, the convicted killer shared even more of the gruesome details of the crime that has him sitting behind bars for life. According to him, his older daughter, Bella, didn’t just ask him, “what are you doing with Mommy?” She also watched him strangle her younger sister, Celeste — and realize that the same was likely going to happen to her.

According to a summary of the investigation shared with the media, Watts admitted that his daughters were still alive when he drove them and their mother’s body out to the oil tank. “Cece was first,” he said. In the backseat of the car, he put a blanket over Cece’s head and half-smothered, half-strangled her with one hand over her mouth and nose and the other over her throat. After disposing of her body in one of the oil tanks, he came back to the car, and that’s when Bella asked, “What happened to Cece?”

Worse, she seemed to realize what was going on. “Is the same thing going to happen to me as Cece?” Bella asked according to her father — but he can’t remember whether or not he told her, “Yes.” Unlike Celeste, though, who was only 3 years old, 4-year-old Bella fought back. Her father revealed in the interview that her last words were, “Daddy, no!” and that she struggled under the blanket, trying to breathe, as he strangled and smothered her, too.

The detail is haunting. But it haunts Watts, too. According to him, he hears his daughter begging, “Daddy, no!” every time he closes his eyes. And though he can’t explain why he did what he did in the moment, he attributes it all to an overwhelming feeling of rage that his wife, Shanann Watts, had allegedly said that she was going to leave him. “If I was thinking, this wouldn’t have happened,” he said. “[If I was aware of] any partial hint of what I feel for those girls and what I feel for my wife, then none of this would have happened. … I wasn’t thinking.”