A 15-year-old girl who fatally shot her 16-year-old brother suffered years of abuse, including being locked in a room for weeks at a time with only a blanket and a bucket to use the bathroom, according to police reports and interviews.

The shooting at a small white house off a dirt road in rural north Florida happened Monday while the children's parents were away for work. The father, a truck driver, and his wife, who often goes with him, left the 16-year-old boy to watch over the 15-year-old, her 11-year-old sister and their 3-year-old sister, police said. The parents left Sunday and were due back Tuesday.

Sometime Monday, the 15-year-old girl was locked in her room by her brother, police said. After the boy fell asleep, she talked her 11-year-old sister into unlocking her door.

The older girl knew her parents kept a pistol in their room, but they had locked their door. So the girl went outside and used a knife to remove an air conditioner from her parents' bedroom window. She climbed in while her 11-year-old sister kept watch and grabbed the gun out of a pink bag and loaded it, police said.

The girl went back inside the house, telling her young sisters to get in the closet, she told police. She turned her head and fired at her sleeping brother in the living room, and he screamed "Help! Help!"

She buried her head in a pillow for a while and upon returning to the living room, the girl found her 3-year-old sister trying to wake her dead brother, according to the police report.

She fled with her 11-year-old sister, leaving the 3-year-old behind, police said.

"It's hard for us to get our arms around this act," Columbia County Sheriff Mark Hunter said. "This is the stuff nightmares are made of."

Police caught up with the girls after a friend of theirs received a "weird phone call" from the 11-year-old girl, saying she had run away and needed someone to pick her up from a Dollar General, according to a police report. When the woman arrived, she found the older sister there, too.

The older girl said something might have been wrong with another sibling at home. As she spoke, she applied makeup and "would not maintain eye contact and appeared emotionless," officers wrote in a police report.

She soon started crying and told the officers that her brother had beaten her and that she had shot him.

When officers arrived at the home, the 3-year-old said: "he's dead." The brother's body was lying near the fireplace, under a blanket with his head on a pillow.

Police have not released a motive for the shooting.

The girls' mother told police that they often locked the 15-year-old girl up when she misbehaved. The longest they kept her locked in her room was 20 consecutive days, the father told police.

In the girl's room, police found only a blanket and a bucket filled with urine in the closet.

"It was learned that (the 15-year-old girl) has made past attempts at ending her life but neither law enforcement nor (emergency management services) was notified," police wrote in their report.

The girls are in being held in juvenile detention on suspicion of murder, and a prosecutor is trying to decide whether they will be charged as adults. Their parents face charges of child neglect and failing to supervise.

The 3-year-old is in the custody of child welfare officials.

Police documents released Wednesday said the girl's uncle was convicted of molesting her in 2010. They also say the children's mother discovered the siblings having sex in 2011. Authorities and child welfare officials investigated, but no one was charged.

Because of the girls' ages and abuse allegations, The Associated Press is not naming the girls, their brother or the parents.