FARGO, N.D. (AP) - Joseph Duncan bought night-vision goggles here two days before renting a car in the Minneapolis area and vanishing, police documents show.

Duncan, a convicted sex offender whose last known address was a north Fargo apartment, was charged this week in Idaho with murder and kidnapping in the bludgeoning deaths of three members of a Coeur d'Alene, Idaho, family found slain May 16.

Two children were abducted from the family home. Shasta Groene, 8, was in Duncan's company when he was arrested July 2 at a Coeur d'Alene restaurant. The remains of Dylan Groene, 9, were found last week at a remote Montana campsite.

Kootenai County, Idaho, sheriff's officers have said in court documents they think the attacker used night-vision goggles to case the house.

Police in Fargo and in Detroit Lakes, Minn., have been retracing Duncan's steps to help with the Idaho investigation. Fargo police said a receipt shows Duncan bought the goggles at a local Wal-Mart on April 13. He rented a car in St. Paul, Minn., on April 15.

"It's concerning," Fargo Police Lt. Tod Dahle said when asked about the Wal-Mart purchase. "But what that means is the tough part."

Police started looking for Duncan after April 18, when he failed to contact a court officer in Detroit Lakes about his whereabouts, as required by law. Duncan was charged March 4 in Becker County, Minn., with molesting a 6-year-old boy, and was released on $15,000 bond.

Fargo and Detroit Lakes police shared information about Duncan, but they found nothing to indicate where he went, Detroit Lakes Police Chief Kelvin Keena said.

"We began to meet on a regular basis to keep each other abreast of what we were doing," Keena said. "You know, I think we did what we could with what we've got."

A search of Duncan's apartment in early May revealed no clues about his whereabouts, Fargo Police Chief Chris Magnus said.

"As these pieces of information come out relating to this investigation, it's tough for people to understand that a lot of these different things you learn in hindsight," Magnus said.

Police made quarterly visits to Duncan's apartment after he was released from prison and moved to Fargo in July 2000, Magnus said. The visits were not required by law, even though Duncan was registered as a sex offender.

Duncan was convicted of sexually assaulting a 14-year-old boy at gunpoint in Tacoma, Wash., in 1980.

In court documents, Idaho authorities allege Duncan took the two children from their home after killing their mother Brenda Groene, 40, their brother Slade, 13, and Brenda's boyfriend Mark McKenzie, 37.

When he lived in Fargo, Duncan was a computer science student at North Dakota State University. He made the dean's list twice and was a member of the Phi Kappa Phi National Honor Society, an academic excellence fraternity.

"Most of the contact people had with him in Fargo was mostly positive," Magnus said. "That includes neighbors and people at NDSU."

Former neighbors at Duncan's Fargo apartment complex have declined comment.

Duncan received money from two people with Fargo ties after he was arrested in Minnesota.

Fargo businessman Joe Crary wrote Duncan a $15,000 personal check to help him make bail, police said. Dr. Richard Wacksman, a Florida pediatrician who once lived in Fargo, told police he gave $6,500 to Duncan to help with attorney fees.