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, investigators said on Tuesday.

Bernalillo County Sheriff Dan Houston told reporters that 15-year-old Nehemiah Griego, who is accused of shooting his mother and siblings before killing his father hours later, had been contemplating the killing spree for "at least a week."

After killing members of his family, Griego told police, he reloaded and planned to go to a nearby Wal-Mart store.

"He contemplated ending this with mass destruction," Houston told reporters Tuesday.

Instead, the teen went to his girlfriend's home and then to a church where his father once worked as a pastor.

A staff member called the police, Houston said.

The murders have convulsed the faith community of Albuquerque, where Greg Griego, the teen's father and a former gang member, worked as a pastor and volunteered with inmates at the Metropolitan Detention Center.

A Statement of Probable Cause described Griego's changing stories to police as well as the gory details of the quintuple homicide that took place on Saturday inside the family's Albuquerque home.

The teen told police he shot his mother, the first victim in his rampage, because he was "frustrated" with her, Houston said.

After shooting his brother and two sisters, Griego, who police said often played violent video games, 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 Sandy Hook, 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 at the family home as "horrific." Continued...