Sigg told detectives what he had told his mother hours earlier: He killed Jessica Ridgeway.
On Tuesday, the 18-year-old Sigg confessed again in Jefferson County District Court, and despite advice from his attorneys, changed his plea to guilty in the kidnapping, death and dismemberment of Ridgeway.
He also admitted to further crimes, including sexually abusing Ridgeway and attacking a female jogger last year on Memorial Day.
Defense attorneys had worked to exclude Sigg's confessions for months, suggesting that since Sigg was 17 at the time, he was too young to understand his actions and decisions.
"There was no plea bargain in this case. There were no concessions," 1st Judicial District Attorney Peter Weir said. "We were not going to make any concessions for Austin Sigg."
Jefferson County District Court Chief Judge Stephen Munsinger accepted the guilty plea and advised him that the murder charge alone carries a sentence of life in prison.
Prosecutors are expected to seek consecutive sentences for the multiple charges to ensure that Sigg is never eligible for parole. His defense attorneys want the sentences to be served concurrently.
Sigg pleaded guilty to a total of 15 charges, including first-degree murder, kidnapping, sexual assault of a child, sexual exploitation of a child and attempted kidnapping.
A two-day sentencing hearing is scheduled for November 18.