(CNN) -- A 14-year-old boy was arrested Wednesday for allegedly killing his father, stepmother and stepsister on a New Mexico ranch owned by ABC newsman Sam Donaldson, authorities said.
Donaldson discovered the crime scene on the property Tuesday afternoon when he went to the home of his ranch manager.
Donaldson immediately notified authorities who found all three bodies in a shallow grave on the property.
Authorities said the murders were carried out while Donaldson was away from the ranch over the July Fourth holiday.
Cody Posey, 14, was arrested Wednesday afternoon and placed in a juvenile facility, said Lincoln County Sheriff Tom Sullivan.
Prosecutors plan to file a petition on Thursday alleging three counts of murder and four counts of tampering with evidence.
Posey is accused of killing his father, Delbert Posey, the manager of Donaldson's ranch; his stepmother; and his 14-year-old stepsister.
It was not immediately clear if prosecutors would seek to try him as an adult.
Sullivan said a search of the area "revealed the bodies of one male and two females in a shallow grave." The bodies were unearthed by the medical examiner's office early Wednesday.
ABC confirmed the details of the investigation and released a statement from Donaldson.
"My wife, Jan, and I are shocked by the apparent triple homicide that took place on our ranch in Hondo, New Mexico, sometime over the Fourth of July weekend while we were away in Santa Fe," Donaldson said.
"We understand the investigation is continuing, and we have been asked by authorities not to comment further at this time."
Donaldson, who grew up on a ranch in New Mexico, owns several thousand acres of land around Hondo, located in the south-central portion of the state, about 45 miles west of Roswell.
Donaldson raises cows and sheep on the property and has often spoken about his fondness of the land and ranching.
