A 15-year old boy has allegedly killed his two younger brothers, aged 4 and 10, in West Point, Utah.

The boys’ mother found her 4-year old dead in their home and ran to find her other children, but they were nowhere to be found. After calling authorities, the body of her 10-year old was found by deputies. The 15-year old had already fled, but he was quickly found by investigators and was later booked into a juvenile detention center. Police say the two younger boys likely died from knife wounds.

Neighbors and people who knew the suspect say they are shocked; the boy was intelligent and did well in school, and was often seen running for exercise around the neighborhood. However, the two younger boys were adopted, and the suspect was the biological child of the mother. Reportedly, three other siblings weren’t home at the time of the murders.

“I’ve talked with their mother many times but I never got the sense that there was this kind of trouble,” says neighbor Lindsey Caballero. “You could tell there was a sibling rivalry between the biological son and the adopted boys. But I never imagined it could go this far.”

Sadly, this isn’t the first case of a sibling being charged with murder this year; a New Mexico teen killed his parents and three younger siblings after depression and suicidal thoughts crept in. And just earlier this month, a California boy admitted to killing his 8-year old sister and blaming it on an intruder.