A 15-year-old girl accused of fatally shooting her brother in Florida Monday was frequently kept locked in her bedroom with a bucket for a toilet, sometimes for days, police said Wednesday.

The 15-year-old is accused of fatally shooting her 16-year-old brother as he slept on the living room floor of their Columbia County home sometime before 10 p.m. Monday. She and her 11-year-old sister, who police say assisted in the crime, are charged with premeditated murder. Prosecutors have not yet decided whether to charge the girls as adults.

"This is really, really a sad case," Columbia County Sheriff Mark Hunter told reporters Wednesday. "For our community to have this … there's going to be a hundred thousand whys. We might not know why the child acted like this."

Police learned of the deadly shooting after a woman called police in nearby White Springs to say she’d gotten "a weird phone call" from the 11-year-old, and that two girls may have run away from home. The 15-year-old eventually told police that her older brother had beaten her and locked her in a bedroom, and she shot him, police said. Sheriff’s officials went to the home and found the boy dead at around 10 p.m.

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The 15-year-old told detectives after the body was found that she had been locked in her room while their parents were away, that she asked the 11-year-old to unlock the door as their brother slept, then crawled through a window to get in their parents’ locked bedroom and retrieve a 9 mm handgun, according to the arrest report.

The 15-year-old told a detective that she told her younger sister to take their 3-year-old sibling and hide in a closet before she shot her brother as he dozed, police said. The children's parents were not present at the time of the shooting; they left at 11 a.m. that day and left the 16-year-old in charge, Hunter said.

The 15-year-old would be locked in her bedroom with a bucket for a toilet as punishment for acting up, police said in an arrest report. The girls' parents said she would be locked in her room about four times a week, sometimes for days. The father said the longest time was 20 days, while the mother said longest was four days, according to cops.

The girls’ parents were charged with neglect because of the conditions at the home, Hunter said. They appeared before a judge via video Wednesday and bail was set at $20,000 each. Both remained in jail Wednesday, according to jail records. The 3-year-old was taken by the state Department of Children and Families. The 15-year-old and 11-year-old are being held at separate juvenile detention facilities, officials said.

Sheriff's officials had been called to the house in 2011 for what was called a "juvenile issue" and it was turned over to the state Department of Children and Families, but which was determined to be unfounded, Hunter said. A spokeswoman for DCF said she could not provide more information, but the agency was investigating.

Because of the abuse allegations and the age of the suspects, NBC News is not naming the defendants.

Veronica Thomas, a pastor in White Springs, was called by a relative to assist in what was first thought to be a case of a runaway child, and she along with police spoke to the 15-year-old. "She was I think in shock. I really do," Thomas said of the 15-year-old. "The emotions were all over the place."

"It’s a bad situation. It’s not something you think happens in a small town like this,” Thomas said. “It’s something you see on the news, something happening at other towns."