A 15-year-old in Utah was arrested Thursday in the deaths of his two younger brothers, ages 4 and 10, police said.
Authorities are treating the case as a double homicide, NBC affiliate KSL reported.
The victims are said to have suffered injuries "consistent with penetrating knife wounds," Davis County Sheriff Todd Richardson told KSL.
"As a result of our investigation, this morning we have taken the 15-year-old juvenile into custody," Sergeant Susan Poulsen of the Sheriff's Office, adding that prosecutors were expected to formally charge the boy on Monday. "We believe he acted alone."
The boys' mother discovered her 4-year-old son dead on the floor Wednesday afternoon upon returning to her West Point home from a dance recital with her three other children, KSL reported. Police later found the second body.
The 15-year-old, who is not being identified because he is a minor, was found late that night wandering the streets about five miles away. He was taken to a hospital for evaluation and minor injuries, officials said.
According to KSL, the teenager has made headlines before, when he ran away from home in 2011. He was eventually found about four miles away.
Ann Durrwachter, a neighbor whose son attends school with the teenager, told KSL the boy was a "model student."
"From what I understand, he's a model student, from what I've heard. I've never had any complaints about him or his family. I always figured him as model 15-year-old that every mom dreamed of having. He was just carefree almost. He just kind of did his own thing as most boys do."
The boys' father is on active military duty. Four of the couple's children were adopted, including the two victims, The Associated Press reported. The 15-year-old is the couple's biological child.
The gruesome act has left the quiet community in shock.
"West Point is just a quiet area. Not that many people even know it exists," Durrwachter told KSL, adding that the family was kind and loving.
"They were definitely a very positive family," she told KSL. "Sweet, sweet family. Our kids played together. They walked up and down our streets, rode bikes."
The arrest comes amid heightened national attention to violence by children following the high profile stabbing death last month of an 8-year-old California girl, Leila Fowler. Her 12-year-old brother has been charged with second-degree murder in her death.
Reuters contributed to this report