(CNN) -- The 13-year-old boy told them he knew wrestling on TV was fake, sheriff's officials in a New Orleans suburb said.
But, according to authorities in Jefferson Parish, his account of practicing "WWE"-style wrestling moves by punching, jumping on and slamming his 5-year-old half-sister to the bed was all too real, and fatal.
Viloude Louis, 5, of Terrytown, was found unresponsive on a bathroom floor Sunday. She was taken to a hospital, where she was declared dead.
The boy, whose name was not released by authorities because of his age, has been accused of second-degree murder; he remained in Rivarde Juvenile Detention Center on Wednesday.
He "was smiling and appeared as though he took great pride in what he was doing," sheriff's Col. John Fortunato told CNN on Wednesday. "He showed no remorse while he was being interviewed in the presence of his mother."
The half-siblings' mother left them at home when she went to the store Sunday, authorities said in a statement.
The juvenile suspect initially said he found Viloude lying in the bathroom, complaining of a stomachache, authorities said. The boy said he took her downstairs, where she lay down on a sofa. He said he called 911 when he noticed she was not breathing, authorities said.
The mother returned home while emergency officials were on the scene.
An external examination of the little girl by the coroner's office noted no obvious signs of trauma or major injuries, and her death was listed as unclassified.
But a follow-up interview with the teen boy and his mother Monday brought a different account of what allegedly occurred while he was babysitting.
The 13-year-old, who is 5 feet, 5 inches tall and weighs 100 pounds, told an investigator about punching his half-sister in the stomach several times and slamming her on the bed, authorities said. The youth said he was practicing "WWE"-style moves on the 5-year-old.
"Additionally, he jumped on the victim and struck her with his elbow several times like the wrestlers do on television," the sheriff's office statement said.
The boy said the victim complained that she was hurting, "but he continued to slam, punch and elbow her for an additional two or three minutes, stopping when his mother called him on the phone to check on he and the victim."
The boy smiled and appeared to enjoy talking about the wrestling moves, authorities said.
An autopsy showed the girl's injuries included severe blunt force trauma to the body, multiple internal injuries, including broken ribs, lacerations to the liver, and internal bleeding.
The Jefferson Parish District Attorney's Office declined to provide further information or comment, noting that the suspect is a minor.
WWE, the Connecticut-based World Wrestling Entertainment, in a statement Wednesday called the girl's death a tragedy. It said authorities determined Viloude's death was "not an accidental death due to a wrestling move.
"The facts of this case clearly point to a lack of parental supervision. ... It is illogical to conclude that the repeated, brutal and ultimately fatal beating of a 5-year-old little girl by a teenager could be confused with imitation of WWE action seen on TV. "
The mother will not face charges, according to Fortunato.
CNN's Joe Sutton and Marlena Baldacci contributed to this report.