FOUNTAIN, Fla. (AP) — The death of a 10-year-old girl, allegedly at the hands of her 15-year-old brother, stunned employees of a Florida sheriff's office who had tried to help the family in the past, an authority with the office said Monday.

"About three or four years ago, one of our deputies discovered the family living in basically a pole barn with walls that were covered with tarps," said Maj. Tommy Ford of the Bay County Sheriff's office. "We went out there and helped to put walls up and brought some food and Christmas presents for the kids."

Deputies arrested the 15-year-old boy on Sunday afternoon after finding his sister's body in a field near the home. The girl was reported missing Sunday morning. The Associated Press does not generally name juveniles accused of crimes.

Ford said the boy became a suspect during the search and that he later confessed to killing his sister. In an arrest affidavit released by the department Monday afternoon, an officer who interviewed the boy wrote that he first told investigators that he shot his sister about 3 a.m. Sunday after mistaking her for a deer. The boy later said he saw someone attack his sister and he accidentally hit her while trying to fire at the attacker. The boy changed his story a third time and said that he accidentally shot his sister while trying to teach her how to shoot his rifle.

The boy also told investigators that he cleaned his sister's body, changed her clothing and moved her to the field where the body was later found.

Ford said prosecutors could charge the 15-year-old as an adult.

Earlier Monday, Sheriff Frank McKeithen described the boy in a news conference as "hardcore," saying he acted much older than 15 when confronted by authorities.