A 15-year-old Florida boy stands accused of fatally shooting of his 10-year-old sister, whose body was found clad in a white sundress and covered in towels, authorities say.

Izabella Heffernan was discovered Sunday afternoon, hours after her family reported her missing in Fountain, Fla., according to the Bay County Sheriff's Office.

Her older brother now faces an open count of murder in connection to her death.

At a news conference Monday, Bay County Sheriff Frank McKeithen said the teen's behavior was unusual for his age.

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"This is not the typical 15-year-old kid you would see every day on the street," McKeithen said, according to the Panama City News Herald. "This is a hardcore kid: He lied, he was deceitful, he screamed, he cursed. He was like talking to a 30-year-old murderer."

During an interview with investigators, the 15-year-old admitted to shooting Izabella, although he said that he had mistakenly thought his sister was a deer when he fired, according to an arrest affidavit. He told the sheriff's office that after he shot the girl with a .22 caliber rifle, striking her in the chest and face, he moved the body and hid the gun.

He later changed his story, according to the affidavit, instead telling authorities that he was trying to shoot someone who was attacking the 10-year-old. He said he had accidentally shot his sister instead.

Finally, the teen told police that he was teaching Izabella to shoot, which was why he had taken her outside early Sunday morning.

"While doing this he stated he accidentally shot her in the chest," the affidavit reads. "He thought it hit her in the lungs and stated, 'She started making gurgling noises, and I did not know exactly how to deal with that, so I shot her again.'"

The 15-year-old told investigators that he moved the body and put Izabella in a white dress, according to the documents.

He also told investigators where to find the gun that he used: in an abandoned home that was nearby.

"I heard a young man say that she was one of those students who, if you were having a bad day, she would come sit by you and talk to you," Izabella's elementary school principal, Chris Beard, told the News Herald. "She always had a smile. She was a very sweet little girl."