Police in the Denver suburb of Westminster took Sigg into custody on Tuesday night after receiving a phone call, from his mother, that led them to Sigg. He was formally arrested Wednesday.
Sigg reportedly told his mother that he was responsible for the murder of the 10-year-old who was abducted while walking to school. Part of her dismembered remains were found five days later in a field at a park. The torso was reportedly found on Oct. 10. Her remains weren't identified until Oct. 12 because of the body's condition.
Police released photos of a cross apparently left behind by Jessica's killer.
"I made the phone call, and he turned himself in. That's all I have to say," his mother, Mindy Sigg, told a reporter before she broke down in tears and hung up.
Investigators say Sigg also admitted to having some of the 10-year old's remains in a crawl space in his home. Authorities reportedly have recovered those remains since Sigg's arrest.
A Denver television station reported that Sigg in his confession told police he didn't receive the joy from fatally strangling her that he thought he would, so he went further.
Sigg attended Witt Elementary - the same school Jessica went to - but he moved on to middle school in 2007, before she enrolled at Witt, Jefferson County Public Schools spokeswoman Lynn Setzer said.
Authorities declined to elaborate on the arrest and have released few details about the case. Court documents have been sealed, but a police custody report said Sigg was cooperative when he was arrested and waived his rights.
Sigg, an Arapahoe Community College student, showed interest in mortuary and forensic science. Friends say he often wore black and hung out in the "goth corner" of the school's cafeteria.
One of Sigg's classmates said he had been bullied for having a high voice. Another friend of Sigg's says he wasn't a big fan of high school.
Childhood friend Elexus Parmitter knew Sigg very well.
"He was a like the outcast. He wasn't popular, he wasn't in any sports. He was just a really smart kid," she said. "I grew up with the kid. He has a great mom, his brother is very smart, his little brother. They're just all great."
CBS4 in Denver said Sigg came from a troubled home that included divorce and many run-ins with the police.
The CBS affiliate also said Sigg's father, Robert Sigg, has a long arrest history, including multiple DUIs, assault, domestic violence and drug-dealing arrests.
Robert Sigg was involved in numerous civil actions and was convicted in 2006 for involvement in a mortgage fraud scheme after he was arrested by the FBI, reports say.
Austin Sigg's mother also reportedly filed for Chapter 7 bankruptcy last May.
The arrest brought some relief to the community, which has been on edge as authorities searched for Jessica's killer. More parents have been waiting with their children at bus stops, and high school students have volunteered to walk younger children to school to keep them safe.
"Every parent in every Colorado community will rest a little easier tonight," said Gov. John Hickenlooper, who has a 10-year-old son. "While we still mourn the death of Jessica Ridgeway, we are relieved an arrest has been made and the pursuit of justice can continue."
Copyright 2012 KCTV (Meredith Corp.) All rights reserved. The Associated Press contributed to this report.
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