The killer said he lured Jamie into his apartment by asking if she wanted to see his pet rat. Once inside, she was hit over the head with a wooden cutting board.
Underwood said he then smothered her with her hands, placed duct tape over her nose and mouth and sliced her neck with a knife in a failed attempt to cut off her head.
Underwood consented to a search two days later during an interview with the FBI. His taped confessed, made hours later, was played for jurors during Underwood's trial.
In the video, Underwood admitted fantasizing of kidnapping a young boy or girl, forcing them to watch pornography, then engaging in torture, decapitation and molestation of the body.
Jurors deliberated just 25 minutes to convict Underwood of murder on Feb. 28. It took them eight hours to choose his fate a week later.