Sam Donaldson on the Slayings at His Ranch

Donaldson called the sheriff's department at 5 p.m. local time on Tuesday after he went to the Poseys. Sheriff's deputies searched the area and found the bodies in a shallow grave, Sullivan said.

Why?

Donaldson said he was told Cody Posey "has made certain admissions, taking the police to a spot on the river behind a field we have where the gun apparently was thrown." Donaldson added: "Whether police have recovered the gun, I don't know."

Donaldson also did not want to speculate on what could have motivated the killings.

"Paul was a little hard on his son, but that's the way in the Southwest," Donaldson said. "I never saw Paul raise his voice or hand in anger to his son, but [he] clearly expected a lot of him … I don't know. I'll let the psychiatrist figure it out."

"But at some point, this young man, if he did the crime as the police are going to charge, snapped," Donaldson continued. "You ask yourself, if he was angry at his father, why did he then, if he did it, kill his stepmother and that beautiful little girl, her daughter, who he clearly got along with?"

Donaldson said he and his wife hired Paul Posey to work at the ranch in October 2001. His ranch is in the south-central part of the state, between Roswell and Ruidoso. He is a native of El Paso, Texas, and his father bought farmland in New Mexico in 1910. Donaldson has added to the property over the years.

Donaldson joined ABC News in 1967 and covered the administrations of presidents Carter and Reagan as chief White House correspondent. He also was the anchor of "World News Sunday" and hosted news programs including "This Week" and "Primetime Live."