He was arrested Thursday in the deaths of the boys, ages 4 and 10, at the family home in a Utah subdivision of new houses and tidy lawns, police said.
"He spoke bluntly with our investigators," said Davis County Sheriff Todd Richardson.
County Attorney Troy Rawlings said he wasn't prepared to file charges. He was trying to find out more about the boy and killings that stunned the community, about 30 miles north of Salt Lake City.
Officials described the older brother as an honor student and a long-distance runner on the track team — when his mother wasn't home-schooling him, while neighbors said he was socially awkward with a speech impediment.
"I'm still in shock," neighbor Karin Jackson said Thursday. "This is a wonderful neighborhood and the kids are usually outside playing."
The younger brothers died from knife wounds following the attack, according to a preliminary report by the medical examiner, and the 15-year-old allegedly acted alone, apparently on an impulse, Richardson said.
At first he was thought to be a third victim, missing from the crime scene, and police publicized his name while looking for him. The Associated Press is withholding his name because of his age.
"There are more questions than answers at this point," Rawlings said. "This teen in custody has a presumption of innocence. Facts are being gathered to assist with critical decisions."
The 15-year-old and his two younger brothers had been left home alone. The family has six children, and police said their mother took the other children to a dance recital, returning to find first one body, then another. Their father, reportedly a Department of Defense engineer, was away in Alabama.
Nobody was at the home throughout much of Thursday, when the home was cordoned off by police tape, and the parents couldn't be reached.
Four of the family's six children are adopted, and neighbors spoke highly of them.
But the 15-year-old was "different," said Scott Green, an ex-neighbor who said he once found him throwing dozens of rocks over a fence onto his trampoline.
The father is an engineer working for the Air Force, Green said. At first, authorities said he was active duty military, but later said they weren't certain about his status with what they believed was the Air Force. The couple had spent time in South Korea before moving to Utah, Green said.
The 10-year-old adopted boy spent a lot of time at his house, playing with Green's daughter — "best of pals," he said.
The 15-year-old was enrolled as a ninth grader at West Point Junior High, member of the National Honors Society and a distance runner on the track team, Davis School District spokesman Chris Williams told The Salt Lake Tribune and KSL-TV.
Williams said the youths' parents moved them in and out of public school over the years, sometimes home-schooling them.
Neighbors interviewed by The Associated Press were unanimous: The 15-year-old kept to himself and wasn't seen except when jogging.
"We never had a history file on him, except for the time he did a runaway," Richardson said.
It was two or three years ago, police and neighbors said. After a 7-hour search, according to the Standard-Examiner of Ogden, police found him four miles away at a Wendy's restaurant, KSL reported.
The sheriff said the 15-year-old had undisclosed, minor injuries when found late Wednesday walking along a street in nearby Layton. The injuries were consistent with having been involved in an attack, said Richardson. He declined to elaborate.
"It's very sad," said Lindsey Caballero, a young mother who lives directly across the street from the suspect's home. "It's scary. It goes to show you never know what's happening."