A teenager accused of stabbing his two younger siblings to death, told investigators he did it so he could be alone in the house, police said.
Officers found the five-year-old girl and seven-year-old boy bleeding from their wounds at Malik Vincent Murphy's family home in Colorado Springs, Colorado.
The 19-year-old had also turned the knife on his father Jefferson Murphy.
O fficer s tri ed to sa ve t he children, who later died after being transferred to a nearby hospital, said police spokesman Lieutenant Howard Black.
He added that Jefferson Murphy survived his injuries but remains in a serious but stable condition in hospital.
Lt Black said Mr Murphy told investigators he was sleeping in his bedroom with his wife when he heard screaming in the basement.
His son had previously talk ed about killing the family, he added.
An arrest affidavit says the teenager, who was held on suspicion of two counts of first-degree murder, bought a knife several months ago with the plan of killing his entire family and burying the bodies in the backyard.
He had been arrested in Illinois in March and accused of setting fire to his parents' sport utility vehicle.
The parents told authorities that Mr Murphy had severe mental health problems and they did not want him prosecuted over the fire, Effingham County State's Attorney Bryan Kibler told the WXEF-FM radio station.
Mr Kibler said authorities agreed to dismiss the case if the teen got treatment.
Neighbour said they were shocked at the killings.
Gilbert Macias told The Gazette of Colorado Springs that he often saw the younger brother and sister playing outside when he got home from work.
Another neighbour, Judy Barnes, told the newspaper that she gave the children sweets when they played outside and they called her "neighbour grandma."
"They were great. They were just kids," she said. "I saw them the day before yesterday. They were playing in the yard with mom and dad." Mr Barnes said the victims were the youngest of Jefferson Murphy's five children, and Mr Murphy was the oldest.
Additional reporting up Associated Press.
