10/01/2013 AT 07:00 PM EDT
A Colorado teen charged with kidnapping and killing
10-year-old Jessica Ridgeway pleaded guilty Tuesday to all 22 charges against him, against the advice of his attorneys.
Austin Sigg, 18, could be sent to prison for the rest of his life for the October 2012 slaying of the girl in Westminster, Colo. Ridgeway was abducted while walking to school, and her disappearance panicked thousands of residents in Denver's western suburbs.
Sigg also pleaded guilty Tuesday to a May 2012 attack on a 22-year-old jogger at a lake in Ridgeway's neighborhood.
He faces a minimum sentence of 40 years in prison with the possibility of parole afterward when he is sentenced Nov. 8. Prosecutors asked Judge Stephen Munsinger to impose consecutive sentences so Sigg spends the rest of his life behind bars.
Sigg cannot face the death penalty because he was 17 at the time of the slaying.
Defense attorney Mitch Ahnstedt told the court that Sigg was entering the pleas against his lawyers' counsel.
The families of both Ridgeway and Sigg were in the courtroom Tuesday. Sigg spoke only to answer the judge's questions about whether he understood what he was agreeing to.
As a prosecutor started to describe how Sigg grabbed Ridgeway and pulled her into his car, Jessica's mother, Sarah Ridgeway, left the courtroom. Defense attorneys asked the prosecutor to stop, saying Sigg already had pleaded guilty.
Sigg's pleas came two days before he was to stand trial.
Ridgeway, a fifth-grader, left home to walk to school in Westminster on Oct. 5, 2012. She never arrived.
Hundreds of people helped search for her. Ridgeway's backpack was found two days later in Superior, a town about 6 miles from her home. Days after that, human remains later identified as Ridgeway's were found in a park.
Sigg, who reportedly wanted to be a mortician
, was arrested and confessed
, police said.