Westminster Police Dept. via AP file This booking photo provided by the Westminster, Colo., Police Department shows Austin Reed Sigg, then 17, after he was arrested in the abduction and killing of a 10-year-old girl.

DENVER — A Colorado teenager who confessed to abducting, killing and dismembering a 10-year-old girl last year pleaded guilty on Tuesday to first-degree murder, sexual assault and also to the botched kidnapping of a female jogger months before the killing, authorities said.

Austin Reed Sigg, 18, pleaded guilty in Jefferson County District Court two days before he was scheduled to go trial for the slaying of Jessica Ridgeway, district attorney Peter Weir said in a statement.

"He has been held fully accountable for his criminal acts," Weir said. "There was no plea bargain. No charges were dismissed as part of a plea bargain."

Ed Andrieski / AP Sarah Ridgeway, the mother of 10-year-old Jessica, is comforted by friends as she sits in on a news conference at the courthouse in Golden, Colo., on Oct. 1.

Jessica Ridgeway vanished on her way to school in the Denver suburb of Westminster last October, setting off a frantic six-day search for the bespectacled fifth grader.

Panicked parents in and around Westminster escorted their children to and from school while the search was under way for the little girl.

Police ultimately found the girl's dismembered remains in the neighboring town of Arvada and also at the home of Sigg's mother in Westminster.

The onetime community college student confessed to killing the girl to his mother, who then turned him into police, prosecutors said at an earlier hearing.

In addition to the murder and sexual assaults charges, Sigg pleaded guilty to kidnapping, robbery, and sexual exploitation of a child.

He also pleaded guilty to attempted kidnapping for the botched abduction of a 22-year-old female jogger at a lake near his home months before the Ridgeway disappearance.

On Monday, Sigg's public defenders contacted the district attorney's office to say their client wanted to plead guilty to all the charges lodged against him, over their objections.

Sigg was 17 and at the time of the killing, but was charged as an adult.

He faces life in prison when he is sentenced at a two-day hearing scheduled for mid-November.