Jurors began debating Monday whether a 16-year-old boy is guilty of murdering his family on newsman Sam Donaldson's ranch, after a prosecutor called the teen a cold-blooded killer who hid the bodies in a manure pile and a defense attorney countered that the shootings were in self-defense.

In her closing argument, prosecutor Janice Schryer urged the jury to convict Cody Posey of first-degree murder in the deaths of his father, stepmother and stepsister despite defense claims that he suffered years of physical and mental abuse.

"It's not a case about abuse," Schryer said. "It's a case about cold-blooded, intentional murder perpetrated by Cody Posey."

Schryer described Posey's parents as loving and having high expectations of the boy, but said the youth didn't like ranch life and "took the opportunity to relieve himself of those expectations and that life."

Defense attorney Gary Mitchell, making his closing arguments at the end of a three-week trial, said the boy had been isolated on the ranch and subjected to repeated abuse, including beatings and excruciating chores, such as filling a trough with water using a Dixie cup.

"It's time the abuse ended for this child," Mitchell said.

Posey was arrested a few days after the bodies of his father and stepmother, Paul and Tryone Posey, and his stepsister, 13-year-old Mary Lee Schmid, were found on Donaldson's Chavez Canyon Ranch in July 2004. The elder Posey was the ranch's manager.

The jury was deliberating first-degree murder charges but also has the option of convicting him of lesser charges, including voluntary manslaughter.

Donaldson was the trial's first witness, taking the stand Jan. 17 to describe finding a bloody scene upon returning from a trip.

On a videotape shown to the jury, Posey tearfully said he shot his stepmother first so she wouldn't call 911, then killed his father, then Mary Lee so she wouldn't tell on him.

Dozens of witnesses testified that the boy was hit, called names and isolated from friends by his family. And on the stand, Posey claimed his father burned him with a metal welding rod after he refused to have sex with his stepmother the night before the slayings.

Prosecutors contend the teen planned the slayings, then lied and tried to turn suspicion away from himself. Prosecution witnesses also testified they never saw a mark on Posey's body.

Mitchell has said the boy — 14 at the time of the killings — told teachers, family and friends about the alleged abuse but the state failed to help him.

"This abuse against this child needs to end and that means the abuse by the state of New Mexico in prosecuting it, too," Mitchell said.