A Florida teen charged in the grisly murder of a 13-year-old classmate, found dead near a pond with dozens of stab wounds, pleaded guilty to murder Monday.

It would have been the first day of jury selection for his trial, court documents show, two years after an early court appearance where he allegedly mumbled about demons taking his soul.

Tristyn Bailey, a cheerleader at Patriot Oaks Academy, was last seen on surveillance video walking with her killer, Aiden Fucci, on May 9, 2021 in St. Johns County. He also attended the school.

An hour later, the same camera captured Fucci walking away alone. 

FLORIDA TEEN ACCUSED OF MURDERING 13-YEAR-OLD TRISTYN BAILEY: ‘DEMONS ARE GOING TO TAKE MY SOUL’

Aiden Fucci frowns with acne-covered face in mugshot

Aiden Fucci, 14, and 13-year-old Tristyn Bailey. Fucci pleaded guilty to murdering her in Florida. (St Johns County Sheriff's Office)

Police later discovered her remains with 114 stab wounds and a piece of the murder weapon broken off in her skull, according to authorities. Her body was about .3 miles from Fucci's house, according to the police report. They found the rest of the knife ditched in a pond nearby and arrested the suspect on May 10.

When they questioned Fucci, he offered changing stories and eventually admitted to having been with Tristyn the night she vanished and getting into an argument with her, court documents show.

Fucci told police he shoved her to the ground and left. When they served a search warrant on his bedroom, they found clothes similar to what he was seen wearing in the surveillance video – with blood on them. 

Trystin Bailey in cheerleader uniform

Tristyn Bailey in an undated photo wearing her cheerleader uniform. (St. Johns County Sheriff's Office)

Fucci, who was 14 at the time and charged as an adult, changed his plea on the same day jury selection was scheduled to begin in his murder trial. Despite his youth, he was listed at the time of his arrest as 5 feet, 11 inches tall and weighing 180 pounds.

Shortly after arriving for jury selection in a first-degree murder trial Monday morning, Fucci's defense entered the guilty plea.

Evidence against him includes more than 100 pages of emails from a school resource officer.

In one, obtained by WJXT-TV, the officer relayed Fucci's friend claimed the slaying was "100% premeditated" and that the teen had spoken about wanting to kill something in the woods.

Read the court notes (App users go here)

MOTHER OF FLORIDA TEEN CHARGED IN STABBING DEATH OF TRISTYN BAILEY TAMPERED WITH EVIDENCE, POLICE SAY

At a prior hearing, Fucci could be heard mumbling, "Please don’t let the demons take my soul. The demons are going to take my soul away," according to WJXT-TV.

Crystal Smith frowns in mugshot

Crystal Smith, the mother of teen murder suspect Aiden Fucci, is accused of trying to wash blood off of his jeans. (St. Johns County Sheriff's Office)

"Why am I here?" he continued. "I just want to talk to my mom and dad. What’s going on? What’s going on?"

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Fucci's mother, Crystal Smith, was also charged with evidence tampering after investigators said they obtained video of her washing blood from her son's jeans in a bathroom sink shortly after they took him for questioning.

Police later obtained the same jeans through a search warrant and allegedly found blood on them and in the sink drain. She has since posted bond and is due back in court on April 4.

After her son's guilty plea, St. Johns County Sheriff Robert Hardwick praised investigators for an "all-out" effort to bring Fucci to justice as well as the community as a whole, which supported the Bailey family and called in tips.

In a separate statement Monday, Baily attorney Matthew H. Hinson heaped praise on prosecutors, the sheriff's office and the family's neighbors.

"Foremost, we would like to give thanks to God," he said in a statement. "Since this nightmare began, we have had the love and support of so many. We know so many have lifted Tristyn, our family, and her friends up in prayer. The members of our community have been simply exceptional in the support they have given us."

Fucci, who remains in custody, is due back in court on Feb. 23.

Fox News' Stephen Sorace contributed to this report.