Late Friday, state and federal agents arrested Peterson on suspicion of murdering Christin on that same night - July 19 - and putting her nude body in a trash bag and tossing it into a Dumpster.

Searchers found Christin's body at the local landfill Wednesday.

Peterson, 22, was being held in the Park County Jail in Cody on Friday pending his initial appearance on felony murder charges.

Christin's family, meanwhile, planned a Tuesday funeral for the girl.

Police listed Peterson's residence as 711 E. Third St., in Powell, a home that he shared with his mother. Authorities said Peterson is separated from his wife, Tyna Peterson. But police and neighbors said he spent much of his time baby-sitting his stepdaughter at his wife's trailer home - trailer number 21 at 639 E. Adams St., known in Powell as the Triangle trailer court.

"I never knew a time when he wasn't there," Smith said.

Park County Prosecuting Attorney Michelle Marker said in a press release that Peterson allegedly killed Christin at that trailer home - little more than 100 feet from the door of Christin's grandparents' home, where the friendly Laramie girl and her family were staying while visiting Powell to attend the Park County Fair.

Christin and her family had lived for a time in Lovell and then lived for about five years in Powell before moving to Laramie about a year ago so that Christin's mother could attend law school at the University of Wyoming. While living in Powell and in later visits, Smith said, Christin had befriended Peterson's 6-year-old stepdaughter, known by the nickname "Andy," who lived at the trailer home across the street from Christin's grandparents.

In June, Christin stayed overnight with Peterson's stepdaughter at the trailer where Peterson so often watched his stepdaughter. And on July 19, Smith said, Peterson's stepdaughter had attended church with Christin's family in Lovell.

Later that same day, Christin disappeared.

"I kept saying I couldn't think of anyone in the trailer court who could do anything like this," Smith said. "But he was the one person who could have said to Chris, 'Andy wants to see you,' and she would have gone over there."

Marker said she is alleging that Christin had voluntarily entered the trailer home on the night of July 19. After Peterson allegedly killed her, Marker said, he placed her nude body into an Army duffel bag, placed that inside a plastic garbage bag and then put put it all into a Dumpster in front of the trailer home.

Marker said prosecutors allege that Christin's murder "occurred in the course of a sexual assault, kidnapping or abuse of a child," which would constitute first-degree murder.

A red and white scooter like the one Christin was riding on the night she disappeared was found at the local landfill the day before the girl's body was found there. The body was in advanced stages of decomposition, Marker said, but had not been mutilated. Evidence so far indicates that no one besides Peterson was involved in the crime or its cover-up, she said.

Peterson is expected to be informed of the charges against him at an initial court appearance today.

Authorities declined to say on Saturday how Christin died or what evidence finally led investigators to Peterson, but Marker said officers had kept Peterson under constant surveillance since he was identified as a strong suspect in the case.

Peterson was arrested after an interview with Special Agent Rich Spencer of the Wyoming Division of Criminal Investigation and Special Agent Todd Dawson of the FBI, Marker said.

Police had searched another trailer home in the Triangle trailer court in the weeks after Christin's disappearance, but the search turned up no leads and had no connection to Peterson, Powell Police Chief Tim Feathers said on Saturday.

He said police had had contact with Peterson before Christin's disappearance but declined to describe the contact or reveal whether Peterson had been charged with other crimes in the past.

A neighbor in the trailer park said residents frequently saw Peterson watching his stepdaughter and a small boy at his wife's trailer. Authorities could not immediately say Saturday whether the boy, who apparently also lived at the trailer home, was Peterson's son or stepson.

"Sometimes you would see him pushing them around town in a stroller," said the neighbor, Chanda Hufford.

She said Peterson had attended Powell High School for at least a short time and generally seemed withdrawn from other students.

"People would try to talk to him, and he just wouldn't say anything," she said.

Reached at her home in Powell on Saturday, Peterson's mother, Shirley Peterson, declined to discuss her son's case.

Peterson's arrest ends the mystery of Christin's disappearance, which had searchers scouring Powell and outlying areas for any trace of the girl or her red and white scooter. Christin had been last seen at about 9:30 p.m. July 19 turning off water to a garden her grandfather maintained about a block and a half from her grandparents' home.

Family, police and neighbors - including Peterson - searched the neighborhood the night of Christin's disappearance and in subsequent days, checking outbuildings, cars and dumpsters that the girl might have crawled into. But the girl's body, wrapped as it was in a duffel and garbage bag, apparently escaped attention.

Tracking dogs followed the girl's scent to a curbside, which suggested to authorities that she might have gotten into a car and been taken from the area. But the truth apparently was much closer to home.

Police knocked on the door of Peterson's wife's trailer, where Peterson was staying on the night Christin disappeared, to ask if he had seen Christin, Smith said. Peterson said no. When police would not tell him the name of the missing child, he apparently spotted people gathering on the lawn in front of Christin's grandparents' and came over to ask who was lost.

"I told him it was Chris," Smith recalled on Saturday. "He want back and grabbed a flashlight and went out to help look.

"So figure that one out."

Later a flier with Christin's photograph and description appeared in the window of the trailer home.

Peterson had occasionally borrowed tools from Christin's grandparents and used their telephone, since his wife's trailer apparently had no phone.

Family and friends had long insisted that Christin was a smart girl who would not have wandered off and gotten lost or fallen into a canal that runs through Powell. Her family launched a campaign to get word out about Christin's disappearance and, with the help of friends, distributed about 200,000 fliers with her photograph and description throughout Wyoming, Montana and the rest of the nation.

"In eight years, she touched more lives that most of us do in a lifetime," her grandfather said.

Christin's funeral is scheduled for 2 p.m. Tuesday at the First United Methodist Church - the largest church in Powell, her father said - to accommodate everyone who knew her and cared about her. He said the service would be open to the public, and the community is invited.

He said he has been receiving e-mail messages from around the world expressing concern for Christin and her family.

"Everyone wants to know what happened to her."