(CBS/AP) GOLDEN, Colo. - Austin Sigg, the teenager accused of kidnapping, killing and dismembering 10-year-old Colorado girl Jessica Ridgeway, is expected to plead guilty Tuesday because he told police he knew what he did was wrong and he wants to give his victim's family some level of comfort.
Sigg is charged with murder, kidnapping and sexual assault in the Oct. 5 disappearance and slaying of Ridgeway in the Denver suburb of Westminster. He cannot face the death penalty because he was 17 at the time of the slaying, although he turned 18 in January. He faces life in prison with the possibility of parole after 40 years if convicted.
While investigators testified that Sigg confessed to strangling Jessica and then dismembering her body in a bathtub, he vehemently denied sexually assaulting her. No semen was found on the girl's body, according to previous testimony in the case.
It's unclear whether prosecutors will drop the sexual assault charges in exchange for a guilty plea to murder in order to avoid a prolonged trial. Prosecutors added the sexual assault charges because authorities said they found child pornography during the investigation.
Prosecutors and defense attorneys won't comment on whether Sigg will plead guilty to all or some of the charges because a judge has issued a gag order in the case.
Sigg also is charged with an earlier attack on a runner in Jessica's neighborhood.
Jessica, a fifth-grade student, disappeared after she left her house to meet a friend just two blocks away so they could walk to school together. Her torso was discovered in a field in the nearby suburb of Arvada, while other remains were found at Sigg's mother's house after she called police.
At a Feb. 23 preliminary hearing, investigators presented 911 recordings in which Sigg told police, "I murdered Jessica Ridgeway. I have proof that I did it."
Westminster police officer Albert Stutson testified that Sigg told him during questioning that he planned to plead guilty. He asked officials if it would give Jessica's family comfort if he were held accountable. Stutson said Sigg told him "he knew what he did was wrong and that he would make up for it someday."
Officers testified that Sigg told them that some of Jessica's remains were hidden in a crawl space in the home of his mother, where he lived. They said Sigg also told them how he abducted Jessica as she walked past his car. He bound her arms and legs with zip ties, placed her in the back seat and took her to his house, where he strangled her, police said.
Hundreds of local, state and federal officers canvassed the Westminster area after Jessica's disappearance, investigating leads and even taking residents' DNA samples. Parents waited with their children at school bus stops or drove them to and from school. The FBI took the unusual step of asking residents to keep an eye out on neighbors - and even family members - and report suspicious behavior.
One resident contacted authorities Oct. 19 to alert them about Sigg because he had a fascination with death, Westminster detective Luis Lopez testified. Sigg was interested in a mortuary science program at a local community college, police said. Two FBI agents responded and took a DNA sample from Sigg on Oct. 19.
Mindy Sigg told investigator Michael Lynch that her son appeared sick and confessed four days after the FBI visit. He opened the conversation by saying he was a "monster," she said.
Mindy Sigg then called 911 to say her son wanted to turn himself in and had confessed to killing the girl. When the dispatcher asked what her son had said, Mindy Sigg replied, "That he did it, and he gave me details, and her remains are in my house."
Sigg also faces an attempted kidnapping charge for the attack on a 22-year-old woman who was running at a lake across the street from Jessica's elementary school. An investigator testified that Sigg used homemade chloroform concocted with a recipe found on the Internet to attempt to subdue the woman.
Jessica lived in Westminster with her mother, Sarah Ridgeway. Jessica's father, Jeremiah Bryant, lives in the Kansas City suburb of Independence, Mo.