ALBUQUERQUE, N.M. - The U.S. teen accused of killing his family and plotting to randomly gun down Wal-Mart shoppers spent much of the day at his church after the early-morning slayings, wandering the campus as dozens of Sunday school teachers were being trained on how to deal with a shooter, a security official said.

But former police officer and Calvary Albuquerque security chief Vince Harrison said it wasn't until hours later that he knew something was wrong.

Harrison, who led the safety training Saturday morning, said he was called back to the church that evening after 15-year-old Nehemiah Griego told a pastor he found his family dead in their home.

"When I met Nehemiah, I knew something wasn't right," Harrison said Wednesday. "I could feel it, I could see it in his eyes and I could see it in his behaviour and his demeanour, so the red flags went up and that's when I called the sheriff's department."

Harrison, who had known the Griego family for about 10 years, said he drove the teen back to the family's rural New Mexico home to meet authorities, interviewing him along the way.

"He went into detail of where they were, where the guns were and he was very matter-of-fact, really cold, so I knew something wasn't right," Harrison said.

The shooting was the latest such attack as the U.S. debates the issue of gun violence and President Barack Obama has made sweeping proposals to Congress that include limiting certain guns and high-capacity ammunition magazines. The issue moved to the top of the president's agenda for his second term after last month's Connecticut school shooting left 20 young children and six adults dead.

After finding the bodies Saturday, sheriff's officials said they took the teen to headquarters. During questioning, he confessed to shooting his mother and three younger siblings in their beds, then waiting in a bathroom with a military-style semi-automatic rifle to ambush his father upon his return from an overnight shift at a homeless shelter.

Investigators said he also told them he had reloaded the family's rifles and taken them with him in the family van with plans to randomly shoot more people.

"That sends chills down my spine," Harrison said. "But obviously, God had a different plan."

Harrison said several people spotted Griego at the church throughout the day and told him hello, thinking nothing of it until his arrest. Officials then reviewed security video and determined the teen had spent much of the day there, he said.

Harrison said he doesn't know why Griego decided to come to the church, but that it was like a second home for the boy, who was homeschooled.

"It was a familiar place to him," he said. "I think if he did have in his mindset to do something foolish and start shooting people there also, I think his demeanour was tamed a little bit because he saw people there he knew."

Sheriff Dan Houston said Tuesday there was no indication Griego intended to harm anyone at the church. The sheriff also said Griego and his girlfriend had spent much of the day together.

A prayer vigil was held at the church Wednesday night.

"Our hearts break, Lord," Pastor Skip Heitzig told the crowd. "We, often in times like these, scratch our heads and wonder why. We are at a loss for words and we are certainly at a loss for explanation."

Relatives, in a statement Tuesday night, said they were heartbroken. They remembered the teen as a bright and talented musician who dreamed of following his family's long tradition of military service and as a boy who accompanied his pastor father on rescue missions to Mexico.

"We have not been able to comprehend what led to this incredibly sad situation," they said. "However, we are deeply concerned about the portrayal in some media of Nehemiah as some kind of a monster."

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Associated Press Writer Jeri Clausing in Albuquerque contributed to this report.

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