Thursday night, their mother found her four-year old dead inside their home. Police say she couldn't find her other two sons when she contacted authorities.
Davis County sheriff deputies later found the ten year old dead inside the home.
"It appears they died from a knife like wound," says Sheriff Todd Richardson.
Richardson says the 15-year old fled the home but Layton police found him shortly after midnight.
"He had some wounds," Richardson says.
After being questioned, Richardson says he was booked into a juvenile facility in Davis County.
"He acted alone," he says. "There are no others at large.
Richardson says their father serves in the military and is stationed in the United States. He says he's been notified and is returning to West Point.
The 15-year old attends West Point Junior High where students were shaken by the tragedy.
"This is really hard to understand" says one student who claims she sits next to him in science. "He is very friendly and always talking about school work and he's very smart.
Early Thursday morning, the bodies of the four-year old and ten-year old were removed from the house and taken to the medical examiner for an official autopsy report. One mother was holding back tears.
"My son is devastated right now," says Angela Moffo. "My son played with the two boys. They were nice kids."
Others say the family was often seeing running as a form of exercise.
"He (15-year old) was a runner," says Ann Durrwachter.
Another neighbor says their father ran marathons.
Lindsey Caballero says she often spoke with the mother of the boys.
"I've talked with their mother many times but I never got the sense that there was this kind of trouble," says Lindsey Caballero.
Caballero did say the two brothers who were killed are adopted and the 15-year old is the biological son. There are also three other siblings who were not home at the time of the deaths.
"You could tell there was a sibling rivalry between the biological son and the adopted boys," she says. "But I never imagined it could go this far."
Police say prosecutors will be screening the case against the 15-year old who will be charged in the juvenile system.