(CNN) -- In a soft voice, teenager Austin Sigg confessed in gruesome detail how he snatched a 10-year-old girl from his Colorado neighborhood and dismembered her.
But what Sigg could not explain to police detectives is why.
"It was a random place, a random time. Random everything," Sigg told officers in interrogation video released this week.
"I was just driving and just kind of looking. And I saw her ... found the place to park. And I parked, and I waited. And she walked by, and I grabbed her."
Sigg is going to have a lot of time to think about it. He was sentenced to life in prison last week. The tapes released this week by the Jefferson County District Attorney give a chilling look into Sigg's thoughts.
Sigg was 17 when he grabbed Jessica Ridgeway off the street in October 2012.
I told her everything would be OK
Sigg told police during the interrogation that he got her into his car. He tied her hands and legs together and drove around for 20 minutes as she frantically screamed and asked him questions.
"I would lie to her," Sigg said. "I would tell her everything was going to be OK. I would just lie to her."
He took the bespectacled, blond girl back to his house. What happened there may have been very graphic. Parts of the interrogation tape were redacted by authorities.
Eligible for parole
Sigg will be eligible for parole after 40 years for the murder. But a judge sentenced him to close to 100 additional years for the other charges against him. Along with Ridgeway's murder, Sigg also confessed to the attack and attempted kidnapping on a jogger that occurred earlier in the year. Using all those charges against Sigg, the judge said he expects Sigg to be confined to a cell for the rest of his natural life.
Turned in by his mother
Jessica Ridgeway was missing for five days and authorities were searching everywhere in October 2012. The break in the case came after Sigg told his mother what he had done. His mother, police said, called 911 and handed the phone to her son.
During the trial, Ridgeway's family members were in court wearing the little girl's favorite color, purple.
"This monster took away from me the light of my life," said Christine Ridgeway, Jessica's grandmother. "It shattered the core of my family."