But it wasn't until Sigg was arrested in the Oct. 5 kidnapping, sexual assault, murder, and dismemberment of 10-year-old Jessica Ridgeway, an attack that horrified and devastated families in the Denver area and across the country, that some friends, upon reflection, saw those behavioral shifts in a darker light.
"When I learned it was Austin, every single memory that gave insight to him doing it flashed into my head like a strobing movie," says Ash, 17, a close friend of Sigg's who asked that his full name not be used because Sigg's friends have been getting harsh comments on Facebook.
Authorities say they have DNA evidence and a confession from Sigg, who, according to reports, directed police to Ridgeway's body parts under his mother's house. On Oct. 30, he was charged as an adult with 17 counts involving the assault on Ridgeway and an attack on a 22-year-old jogger who got away.
But little has been offered so far to explain what may have driven Sigg to commit such a horrifying crime. Experts say that it is rare for the murderer of a child to also dismember the victim, rarer still when the perpetrator is only a teen.
Westminister Police Dept.
But friends say they saw a change in Sigg about two years ago. "He became distant, more aggressive, more antisocial, and more dominative," says Ash, 17.
Austin Caisse, 17, a classmate since elementary school, says they frequently argued over such silly things as who was the cooler Austin or who had the better knife collection, but Caisse always figured Sigg "was more bark than bite."
But he stopped talking to Sigg roughly a year ago, when Sigg seemed to become even more of a hothead amid problems at school and at home.
Other students picked on Sigg over his dark choices in clothing, his voice and his sometimes-abrasive personality, says Caisse. Sigg’s father has been in and out of legal trouble and his mother had financial problems and a history of divorce, reports CBS4 in Denver.
Sigg dropped out in his junior year and got his GED so he could go to Arapahoe Community College "because he was pursuing his desire to be a mortician," says Caisse. The school confirms Sigg is a student there, but says he hadn't yet taken enough classes to enter the mortuary sciences program.
And to get his GED, Sigg attended nearby Warren Tech, where he showed an interest in forensic science and, in March, placed second in the CSI division of a statewide leadership conference.
A few months later, when Jacqueline Miller shared a Psychology 101 class with Sigg at Arapahoe, she recalls Sigg seeming particularly intrigued with her case study on serial killer Ted Bundy, she told The Denver Post.
She says that as she recalled how Bundy had no empathy for his victims, Sigg laughed to himself. "He was so lit up about what I was saying," Miller said. "He was asking about it. He was intrigued. It was almost like he was excited about it."
Sigg's defense attorney has not returned calls seeking comment. The only Sigg family member to comment, his father Robb Sigg, would only say that his "thoughts and prayers are with the Ridgeway family."