Warning: Distressing content

Chad Doerman, the US man who executed his three young sons with a rifle, reportedly confessed to authorities that the unspeakable act had been planned for months.

WCPO news reported that court documents stated the 32-year-old “confessed to planning and carrying out the deaths of victims involved for several months”.

He admitted to lining them up before executing them with a rifle, prosecutors said Friday.

Doerman is also accused of wounding the boys’ mother at the family’s home, shooting her through the hand when she tried to stop his bloody rampage.

Clermont County’s chief prosecutor of Municipal Court, David Gast, said during Doerman’s arraignment on Friday that one of the boys tried to flee into a nearby field but Doerman “hunted” his son down and brought him back to their home before killing him.

Doerman’s bail has been set at US$20 million. Court records did not indicate whether he was represented by a lawyer at his arraignment. He is currently being held in Clermont County Jail.

Gast called the killings the worst crime he’d ever seen.

Deputies responded to the home in Monroe Township shortly before 4.30 pm on Thursday, local time, after receiving a pair of 911 calls, one apparently from the mother who was screaming that “her babies had been shot” and another from a passing motorist who said a girl was running down the street saying her father was killing people, a news release from the Clermont County Sheriff’s Office said.

“The trauma that this man has inflicted on his family, community, law enforcement, first responders, and all the rest of us is unspeakable. There has been a full admission in this case, the case is still new we’re still discovering facts. But the evil horror of what we know is impossible to process,” Gast told the court.

“In an act of incomprehensible cruelty the father that stands before you lined up his three young boys and executed them in his own home with a rifle. In an act of desperation the mother at some point grabbed the gun the father was wielding to attempt to protect them.”

The deputies found three boys, ages 3, 4 and 7, outside the home with gunshot wounds and tried to save their lives, but the children died at the scene.

“They held these children knowing there was nothing they could do,” Gast said. “How do you unsee that sort of abomination?”

“This was the man that every day they woke up looking to for protection, love and guidance in all things, ...” Gast said. “He was their world, he was their guardian and he executed them in cold blood.”

Neighbours in Monroe Township, which is about 120km west of Columbus, reported hearing several shots fired in what they said is usually a quiet, calm neighbourhood.

“I was sitting in the garage, and all of a sudden, I hear ‘boom, boom’ and like five more, and I was like, ‘That’s seven shots,’” Alexis Spoonamore told WLWT-TV in Cincinnati. “I’m shaking. It was a lot. It was bad.”

‘He hid a lot from me’

Doerman’s father has spoken out, saying he is “still trying to comprehend” how his son could have committed the crime.

Keith Doerman, 59, told the Daily Mailthat he believed, despite his son’s admission the killings were planned, that his son “just snapped”.

Doerman Sr said that he had not been able to speak with his son and was seeking answers as to why his son slaughtered his grandchildren.

“There was something going on in his life that he couldn’t handle no more. I can’t talk to him, they aren’t letting me talk to him so I don’t have any answers. He probably hid a lot of stuff from me,” he said.

He also claimed his son was a “fun-loving guy” and insisted that he did not have a criminal record.

However, records show that Chad Doerman was charged in a 2010 domestic violence incident - for choking his father.

- Additional reporting, AP