(CNN) — A 15-year-old Utah boy was arrested in connection with the slayings of his two younger adopted brothers, ages 4 and 10, who were apparently stabbed to death in their home, authorities said Thursday.

The oldest brother — the eldest of six children — was a natural child of the family and was baby-sitting his two younger siblings while the mother was out of the home with the other children, Sheriff Todd Richardson told reporters Thursday. The father, who’s in the military, was in Alabama.

The mother called 911 Wednesday evening after finding her 4-year-old son dead in the home in West Point, Utah. Police then found the 10-year-old son dead in the lower part of the house, but they weren’t sure whether the oldest child was a victim or a suspect, Richardson said.

Hours later, near midnight, police found the oldest brother miles away walking with blood stains on the street. The boy spoke “pretty bluntly” with investigators, who are trying to determine a motive, the sheriff said. He wasn’t carrying any weapons, though police have found two weapons believed to have been used in the slayings, Richardson said.

The 15-year-old boy is facing two homicide charges, the sheriff said.

“He had been interviewed by one of our detectives, and there has been things that have been brought forth that is part of an active investigation, and as time goes on, it will be released as to why he did what he did,” Richardson said.

“I don’t want to speculate what was going through his mind, but he is a natural child of the family and the two children who were victims in this were both adopted,” the sheriff added.

The 15-year-old boy once ran away from home, and in that incident, the family apparently received assistance from a church leader, the sheriff added.

“On that one prior time, he did a runaway, and we located him, so we don’t have a longstanding history file on him,” Richardson said.

The family “did an intervention through their ecclesiastic leadership, and as far as anything through law enforcement or the state, it’s my understanding that they didn’t” do any more, Richardson said.

Prosecutors have yet to file charges or decide whether to charge the youth as an adult, said Sgt. Susan Poulsen with Davis County Sheriff’s Office.

The boy’s name wasn’t being released because he is a juvenile, authorities said.

The father, believed to be in the Air Force, was returning home, Richardson said.

During the initial search of the family home, deputies found the body of the 10-year-old boy, who like his younger brother, had “traumatic injuries,” the sheriff’s office said.

“Obviously the mother is very distraught,” Richardson said.

Just before midnight Wednesday, investigators from several jurisdictions found the oldest son in the Layton area, about eight miles from West Point, and he was taken to the hospital for the evaluation of minor injuries, authorities said. He is being held in Farmington Bay Youth Detention Center.

“We believe the suspect acted alone, and no other suspects are at large,” the sheriff’s office said in a statement.

Both boys’ “manner of death appears to be consistent with penetrating knife wounds,” the sheriff’s office said. The medical examiner’s office has yet to determine the cause of death.

The sheriff couldn’t detail the wounds Thursday.

“I’ve been in this job for a long time, so the things that look like defensive wounds turn out not to be,” said Richardson, who has worked in law enforcement and as a paramedic for more than 25 years.