A teenager accused of kidnapping and killing 10-year-old Jessica Rideway, and attacking a runner will be prosecuted as an adult.

The judge presiding at a hearing for 17-year-old Austin Sigg Tuesday said the defense had waived its right try to move the case to juvenile court. Prosecutors charged Sigg as an adult last month but Sigg's lawyers had the option of asking Judge Stephen Munsinger to move the case to juvenile court.

Munsinger also agreed to move Sigg to adult jail at the request of the state's juvenile justice system. Officials said Sigg is being held in an isolation cell not meant for long-term stays and requires full-time supervision.

Sigg turns 18 in January and Munsinger noted that he would have been moved to adult jail after that anyway.

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Sigg was arrested after sister station KCNC says he confessed to his mother, who then called police. Sigg's mother told the Associated Press that her son turned himself in.

Jessica Ridgeway was abducted on October 5 while walking to school in Westminster, a Denver suburb. Her body was found in an open space just a few miles from her home five days later. The remains were "not intact" according to authorities.

Sigg is also accused of attacking a 22-year-old runner at Ketner Lake, just a half-mile away from Jessica's home. The victim said a man grabbed her from behind and put a rag reeking of chemicals over her mouth. She managed to get away and was able to provide police with a description of the assailant.

Sigg lived just a mile from Jessica's home, and attended the same elementary school, Witt, when he was in the fifth grade.

Reaction among those who knew Sigg was mixed; many former classmates and neighbors said they were stunned, while others described a boy with a troubled background and an infatuation with death.

"I used to play with him when I was little,” Brooke Olds said Wednesday. “He was always like really weird. We didn't ever like to play with him."

"He was always one of the kids in class who was really smart but was always by himself,” Lucas Goodrich said. “He talked to himself, and he just acted really awkward around people."

Sigg dropped out of Standley Lake High School in Westminster, but later obtained a GED. At the time of his arrest, he was studying forensics at Arapahoe Community College, a subject that first piqued his interest in high school. He reportedly told other students that he wanted to become a mortician.

Police called the arrest "a significant step towards justice for Jessica."