"The motive, as articulated by the suspect, was purely that he was frustrated with his mother," Bernalillo County Sheriff Dan Houston said. "He did not give any further explanation."
Houston said Griego had planned the shootings for at least a week, but it's unclear if he ever actually went to a Wal-Mart or why he changed his mind about continuing the attack, which occurred the same day 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 being held just after President Barack Obama unveiled a sweeping package of federal gun-control proposals.
Griego told detectives he also contemplated killing his girlfriend's parents, Houston 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 are 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 Tuesday news conference. Griego apparently had told others of his plans, but whom and when was still under investigation, Houston said.
The teen waived his right to arraignment in adult court Tuesday on charges of murder and child abuse resulting in death and a judge ordered him 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 victims 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. All appeared to have gunshot wounds to the head.
According to Houston and charging documents, it all began early Saturday at the family's home, when Nehemiah Griego — angry and annoyed with his mother — acted on what he described to investigators as homicidal and suicidal thoughts.
Houston said the teen shot his mother while she slept at about 1 a.m. with a .22 caliber rifle the parents kept in a closet. He said he killed his siblings after they woke up and became upset, then grabbed a military-style assault rifle his parents owned and waited in the downstairs bathroom to ambush his father as he returned from work at a rescue mission around 5 a.m.
Griego told authorities he then reloaded the two guns and put them in the family van.
Houston said he didn't know if Griego actually went to a Wal-Mart, but officers found the two rifles, as well as at least a dozen rounds for the .22 and a handful of rounds for the .223 caliber assault rifle in the van.
Griego spent most of Saturday with his girlfriend and her family, Houston said. 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 initially told arriving officers he had come home Saturday morning after spending time at a friend's house to discover his family dead, 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 and alcohol did not appear to play a factor, Houston said. He did note, however, that the teen liked violent video games, including "Modern Warfare" and "Grand Theft Auto." He did not say whether he believed the games were a factor.
Greg Griego was a gang member-turned pastor who had once 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.
A records check by the Children, Youth and Families Department indicated no problems with the Griego 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, there were two 12-gauge pistol-grip shotguns in the home, Houston said. Griego's father taught him to use guns, and they shot together on a regular basis, Houston said.
The home had a security-style sign outside saying "Home Protected by Smith & Wesson."