ALBUQUERQUE, N.M. (AP) -- For at least a week, authorities say, 15-year-old Nehemiah Griego had been planning the attack.
After shooting his mother and three siblings in their beds, he ambushed his father as the pastor returned home from an overnight shift at a homeless shelter. Then the teen reloaded the family's rifles.
His plan was to randomly shoot people at a Wal-Mart on Saturday, which happened to be "Guns Across America" day, until he could be killed in a shootout with law enforcement, according to authorities.
He also contemplated killing his 12-year-old girlfriend's parents, Bernalillo County Sheriff Dan Houston said Tuesday.
And while Griego loaded guns and ammunition into the family's van, Houston said, it was unclear whether the teen did go to a Wal-Mart or how seriously he contemplated continuing his rampage on the same day that thousands of gun advocates gathered peacefully at state capitals around the country to rally against stricter limits on firearms.
The "Guns Across America" events were held after President Barack Obama unveiled a sweeping package of federal gun-control proposals.
What authorities know, the sheriff said, was that Griego texted a picture of his dead mother to his girlfriend, then spent much of Saturday with her and her family, authorities said. That evening, he went to the church where his father worked and confessed to killing his parents and three younger siblings, authorities said.
The sheriff said he didn't know if Griego's contact with his girlfriend avoided further bloodshed. But he said she apparently knew what had happened, and officials were investigating whether she should be charged with failing to report the crime.
"We know Nehemiah had been contemplating this for some time," Houston told reporters at a news conference. Griego apparently had told others of his plans, but whom and when were still under investigation, Houston said.
The motive, Houston said, "was purely that he was frustrated with his mother. He did not give any further explanation."
The teen waived his right to arraignment in adult criminal court Tuesday on charges of murder and child abuse resulting in death and was ordered held without bond. He was arrested Saturday at his family's home in a rural area southwest of Albuquerque.
The sheriff's office identified the dead as Greg Griego, 51, his wife, Sarah Griego, 40, and three of their children: a 9-year-old boy, Zephania Griego, and daughters Jael Griego, 5, and Angelina Griego, 2.
According to Houston and charging documents, it began early Saturday at the family's home, when Nehemiah Griego acted on what he described to investigators as homicidal and suicidal thoughts.
Houston said the teen shot her as she slept at about 1 a.m. with a .22-caliber rifle the parents kept in a closet. He said he killed his siblings and then grabbed his parents' .223-caliber rifle and waited downstairs to ambush his father as he returned from work around 5 a.m.
Griego told authorities he then reloaded the two guns and put them in the family van. Houston said officers found the rifles, as well as at least a dozen rounds for the .22 and a handful of rounds for the other rifle, in the van.
At about 8 p.m., Griego went to Calvary church and told church members that his family was dead. Church officials called 911 and took Griego to his home.
Griego told detectives he discovered his family dead after returning from a friend's house, court documents say. The teen later confessed to shooting his mother because he "had anger issues" and was annoyed with her, the documents say.
The teen had no history of mental illness, and drugs or alcohol did not appear to be a factor, Houston said. He noted that the teen liked violent video games such as "Modern Warfare" and "Grand Theft Auto." He did not say whether he believed the games were a factor.
The boy's uncle, Eric Griego, said in an emailed statement on behalf of the family that the teen is "bright, curious and incredibly talented" with a love of music and sports.
"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.
Greg Griego was a gang member-turned pastor who had served at Calvary, one of Albuquerque's largest Christian churches. He had an extensive arrest record from his gang days, but was best known throughout the law enforcement community for his work as a voluntary chaplain.
Calvary Pastor Skip Heitzig said in a statement Tuesday that the deaths have stunned the church community. "We are doing what we can as a church body to minister to the remaining family members," he said. "Only the Lord Jesus Christ can heal this type of pain and heartache."
A records check by the Children, Youth and Families Department indicated no problems with the family and that Nehemiah Griego had never been in trouble with the law.
Sheriff's Deputy Aaron Williamson confirmed there was no history of any emergency calls to the home in the recent past.
"This is beyond any human reasoning or understanding," Houston said.
"It's horrific. What other words do you use? This is certainly the first time that I have been into a crime scene with this much destruction at one home."
In addition to the two rifles, two pistol-grip shotguns were in the home, Houston said. Griego's father taught him to use guns, and they shot together on a regular basis, Houston said.
Eric Griego's statement said the family had "differing views on gun rights and gun control" and asked the media "to not use Nehemiah as a pawn for ratings or to score political points.
"He is a troubled young man who made a terrible decision that will haunt him and his family forever."
Associated Press writer Jeri Clausing in Albuquerque, N.M., contributed to this report.