He won awards in student crime scene investigation competitions.
His father, on the other hand, had a history of committing them.
Sigg's parents are divorced.
The local district attorney said Sigg, a student at Arapahoe Community College, will be tried as an adult. His first court appearance is set for Thursday.
Along with charges related to Jessica's death, Sigg is also accused in the May assault of a 22-year-old woman on a hiking trail.
One woman shocked by all of this was Sigg's ex- girlfriend, who spoke to a local reporter but only under an assumed name.
"The Austin I knew would never have done anything like that. He was my first boyfriend. He was my first kiss. He was my first date," said Danni. "He was so sweet and so kind. I know that heart is still in him."
But in his neighborhood, Sigg was known more for his penetrating stare.
It was so cold, Emily Alexander said, that it bothered her daughter for days.
"She was freaked out by a teenager staring at her at the park," Alexander said. "After Jessica was kidnapped she told me she knew who it was.
"She was sure she knew who it was and it was this teenager. Because he was 17. I didn't believe her."
Now her daughter, like many in the neighborhood, find themselves once again facing that stare -- from Sigg's mugshot as it blares from television newscasts.
And once again, feeling uneasy.
™ & © 2012 Cable News Network, Inc., a Time Warner Company. All rights reserved.