By Zelie Pollon
ALBUQUERQUE, New Mexico (Reuters) - A 15-year-old accused of killing his parents and three younger siblings over the weekend planned to continue his rampage at a local Wal-Mart store then die in a shootout with police, authorities said on Tuesday.
Bernalillo County Sheriff Dan Houston told reporters that Nehemiah Griego had been contemplating the killing spree for "at least a week."
Houston said Griego told police that after killing his family at their Albuquerque home on Saturday, he reloaded and planned to go to a nearby Wal-Mart store. "He contemplated ending this with mass destruction," Houston said.
Instead, the 15-year-old spent time with his girlfriend, and the pair later went to a church where his father once worked as a pastor. A staff member called police, Houston said.
The murders have stunned Albuquerque. Greg Griego, the teenager's father and a former gang member, worked as a pastor and volunteered with inmates at the Metropolitan Detention Center.
The teenager told police he shot his mother, the first victim in his rampage, because he was "frustrated" with her, Houston said.
Police said that after shooting his brother and two sisters, Griego then waited five hours for his father to return from work and ambushed him with an AR-15 assault rifle - the same type of weapon used in the Newtown, Connecticut, elementary school shootings.
"It's the first time I've been to a crime scene with so much destruction in one home," Houston said, describing the scene as "horrific."
The dead have been identified as 51-year-old Greg Griego, his 40-year-old wife, Sarah Griego, and three of their children: a 9-year-old boy and two girls, ages 5 and 2. The couple had 10 children in all, including from a former marriage.
Police said that after the killings, Griego sent his 12-year-old girlfriend a picture of his mother's body and the two then spent the day together, possibly planning to kill her parents. Charges may also be brought against the girlfriend, Houston said.
Surviving members of Griego's family said in a statement on Tuesday they were heartbroken by the "senseless tragedy" but expressed deep concern "about the portrayal in some media of Nehemiah as some kind of a monster."
"It is clear to those of us who know and love him that something went terribly wrong. Whether it was a mental breakdown or some deeper undiagnosed psychological issue, we can't be sure yet," the statement said.
Griego was due to be charged on five counts of murder and three counts of abuse of a child on Tuesday, but the hearing was postponed after a defense motion to waive his appearance. He remains in custody, said Kayla Anderson, a spokeswoman for Albuquerque's District Attorney's Office.
Anderson said he would be tried as a "serious youthful offender," which means he will be treated as an adult, although the judge has "more discretion" in terms of sentencing.
(Editing by James B. Kelleher, Cynthia Johnston and Eric Beech)