"We believe Joseph Duncan is the only one responsible for these crimes," Kootenai County Sheriff's Capt. Ben Wolfinger told a news conference.
Joseph Edward Duncan III was arrested Saturday at a local restaurant with Shasta Groene, believed to be the sole survivor of the May 16 attack.
The terrible truth about Duncan is now becoming apparent,
Court documents going back to 1980 show that Duncan's sex crimes began when he was a teenager, Blackstone reports. At 17 he pleaded guilty to rape and was put into a therapy program for sex offenders.
He told doctors he was just 12 when he sexually assaulted a five-year-old boy. By age 16 he said he had raped 13 different boys.
Doctors evaluating Duncan said "the danger he presents to children is very great," and "he is not safe to be at large."
After 22 months in therapy he dropped out. His doctors reported: "Mr. Duncan has shown an unwillingness to modify his sexually deviant behaviors."
Duncan spent almost 18 years in prison. When he was released in 2000 he moved to North Dakota and, as required, registered as a sex offender. But neighbors didn't know his history.
In 2004, on a Minnesota playground, he was arrested for molesting a six-year-old boy. He was released on bail because he had no recent criminal record, reports Blackstone.
He skipped bail and was a fugitive until his arrest in Coeur d'Alene.
Duncan has only been charged with kidnapping, and Wednesday marks the first time authorities have said they believe he is responsible for the bludgeoning deaths of Shasta's mother, older brother and mother's boyfriend.
The kids' father, Steve Groene, held a news conference Wednesday in which he said Shasta is home and doing well.
Steve Groene said his daughter is "very upbeat" and "seems to be pretty healthy." And he said she's "really glad to be home."
As for her nine-year-old brother, Groene noted that police believe some remains found in Montana are Dylan's. But he said until someone determines that for certain, he'll still believe his son is alive.