Updated

Statements made by a Colorado teen who killed and dismembered a 10-year-old girl show he was "sadistic," had no empathy for her and had a history of sexual deviance, a psychologist testified Monday.

The testimony came during a sentencing hearing for 18-year-old Austin Sigg, who pleaded guilty to kidnapping, sexually assaulting and killing Jessica Ridgeway in a western Denver suburb in 2012.

Sigg faces a possible life prison term. He can't be executed because he was 17 at the time of Jessica's death.

The defense asserted that Sigg's mother inhaled paint while pregnant with him, and that trauma he suffered before and during his birth left him with head and intestine deformities.

Psychologist Anna Salter of Madison, Wis., who studies sex offenders and was hired by prosecutors, reviewed police interviews with Sigg but didn't speak to him herself.

More On This...

She said Sigg's attack on the little girl was planned, not impulsive.

"He certainly had no empathy for Jessica Ridgeway," Salter said, citing gruesome details from the police interviews. Salter did not say whether she viewed video of the interviews or read transcripts.

Salter also said Sigg did Internet searches on torture, rape and child pornography.

Defense lawyers repeatedly objected to her testimony, calling it "guesswork." Defense attorney Katherine Spengler pointed out that Salter only reviewed records provided by prosecutors and was unaware of some elements of his background.

Spengler said Sigg's mother often inhaled paint in the late stages of her pregnancy and fell down the stairs once shortly before he was born.

Spengler said Sigg was born with an abnormality in his intestines, and that his birth by vacuum extraction also caused him to have a head deformity. She said he had several surgeries before he turned 6.

Salter said she was under the understanding that Sigg's birth was uneventful. She also rejected Spengler's suggestion that bullying played a role in the crime.

Sigg faces a possible life prison term. He can't be executed because he was 17 at the time of Jessica's death.

Sigg, dressed in a blue checkered shirt and khaki pants, sat with his back to the gallery during Salter's testimony. He was led into the courtroom in handcuffs, but the cuffs were removed before the hearing started.

Jessica, a fifth-grader, was abducted while walking to school on Oct. 5, 2012. Human remains identified as hers were found five days later in a park, and more of her remains were hidden in a crawl space at the home of Sigg's mother, where he lived.

Hundreds of people in Denver's western suburbs searched for Jessica, a member of a pewee cheerleading squad who was looking forward to being a zombie lifeguard for Halloween.

Authorities urged residents to watch for any suspicious changes in their neighbors' behavior. Police guarded crosswalks and photographed cars in the area. Parents throughout the region escorted their children to and from school.

A resident contacted authorities on Oct. 19, 2012, to alert them to Sigg because he reportedly had a fascination with death. FBI agents took a DNA sample from the teen. Days later, Sigg's mother called 911, saying her son wanted to confess.

Investigators said Sigg told them he used his hands to kill the girl before he dismembered her body in a bathtub.

Sigg could be eligible for parole after serving 40 years in prison on the first-degree murder charge. But prosecutors want District Judge Stephen Munsinger to impose consecutive sentences on some of the other charges involving Jessica so that Sigg spends the rest of his life in prison.

Sigg also acknowledged attacking a 22-year-old jogger at suburban park in May 2012. In that case, investigators said he used homemade chloroform to try to subdue the woman, who escaped.