"Nothing led up to this. … There are big holes we have to fill," he said.
Neighbors in the quiet, well-kept middle-class neighborhood were stunned by the murders and the news that the teen, who was described as an academically gifted ninth-grader, could have committed such a crime.
"It's sad. It's very sad," said neighbor Lindsey Caballero, who lives across the street.
Richardson said bodies of the boys, a 10-year-old and a 4-year-old, were found about 7:35 p.m. Wednesday with what appeared to be "penetrating knife wounds."
Deputies rushed to the home at 120 S. 1660 West after the boys' mother called 911 to report finding her 4-year-old son dead, and her 15-year-old and 10-year-old sons missing.
Deputies arrived to find the 4-year-old boy dead on the floor of the home. Officers found the 10-year-old's body a short time later in another part of the house.
The 15-year-old boy had been put in charge of the 10-year-old and 4-year-old boys when their mother took her other children to a dance recital at about 4:45 p.m., investigators said.
The Salt Lake Tribune initially reported the name of 15-year-old when he was the subject of a short-lived missing person's alert. However, following his arrest, and consistent with the newspaper's policy of not naming juvenile criminal suspects, his name is now being withheld.
The teen was found walking on the street by Layton police about 11:30 p.m., some eight miles away from the crime scene. He was taken to the hospital to be checked out before questioning. He was then booked into at the Farmington Bay Youth Detention Center on suspicion of two counts of homicide, Richardson said, adding, "We believe he acted alone."
Davis County Attorney Troy Rawlings said Thursday that his office was working with sheriff's authorities to screen charges against the teen, and to decide whether to seek charges against him as an adult.
"We grieve for the family involved," Rawlings said. "There are more questions than answers at this point. This teen in custody has a presumption of innocence. Facts are being gathered to assist with critical decisions."
Davis County Sheriff's Sgt. Susan Poulsen confirmed that the family is military, and that the father has been out of town on active duty. Police did not release any other information on the family.
Davis School District spokesman Chris Williams said the 15-year-old was enrolled as a ninth-grader at West Point Junior High. The 10-year-old attended fourth grade at Lakeside Elementary School. Grief counselors were at both schools Thursday morning.
Williams said the youths' parents would move them in and out of public school, so he didn't know how well other students might have known them. Sometimes they'd be enrolled in public school and at other times they'd be home-schooled, he said.
Williams said the teen was an honors student, a member of the National Honors Society and a distance runner on the track team.
Thursday night, he was supposed to receive some academic awards at a school ceremony, Williams said.
Caballero said there were six children in the home: the teen in custody and his younger brother, who are the couple's biological children; the 10-year-old boy, who was adopted at birth; and the 4-year-old boy and his two younger sisters, who were recently adopted from the foster care system.
She was shocked by the accusation that the older boy was involved in the children's deaths.
"This is just beyond my imagination of what would happen," she said.
Other neighbors watched as detectives worked the scene behind yellow police tape. Someone began a memorial of flowers and balloons near the home's mailbox.
Neighbor Ann Durrwachter, 23, lives two doors down from the victims' home. She said she found it hard to believe the teenaged boy could be involved in the slayings, describing him as well behaved, a model student and a good kid.
"He took good care of his family," she said. "They were an amazing family the family you would go to for a grain of salt and a cup of sugar. The mother is phenomenal. ... They are very loved. We mourn their children as much as they do."
Tribune reporter Jessica Miller contributed to this story.
Certifying youth as adults
Utah law allows charges against youth who are 16 and 17 years old to be direct-filed in adult court if they have committed certain crimes. But because the youth in this case is 15, it would be up to a juvenile court judge to determine whether he should be tried as an adult if prosecutors file to certify him as an adult. During a special hearing in juvenile court, the judge would consider such factors as the youth's age, the nature of the offense and the likelihood of rehabilitation.