The search for 16-year-old Hannah Anderson and her suspected abductor, 40-year-old James Lee DiMaggio, has spanned three states and thousands of miles.
But now that law enforcement officers are at their closest yet to finding the pair, they face perhaps the most challenging search area of all.
The Frank Church River of No Return Wilderness is the largest roadless area in the lower 48 states, sprawling across central Idaho and reaching north to the Montana border. To call the mountainous terrain rugged is an understatement.
About 150 FBI agents converged on the region Saturday morning, joining roughly 100 law enforcement officers from the U.S. Marshal's Service, Idaho State Police, Valley and Ada County sheriff's offices, the San Diego Sheriff's Department and other agencies.
"We are parents. If this was our child, we'd want the same resources out there," said Jason Pack, an FBI special agent from the agency's national press office in Washington, D.C. "We'll be here as long as it takes."
The search is technically and logistically complicated. Because authorities believe DiMaggio to be armed and dangerous, the local search and rescue team volunteers that know the area best aren't being used.
"The suspect is believed to be armed, so you can't have untrained folks out there. They have to have law enforcement training," Pack said.
The Frank Church River of No Return Wilderness Area is roadless, which means federal law prohibits motorized vehicles from going in. Instead, searchers are hiking or riding horseback, aided by trained search dogs which may be able to track the missing pair. There are a few airstrips in the 300-mile search area, where fixed wing aircraft and helicopters can land.
"It's called the River of No Return for a reason," said Mike Medberry, a 57-year-old writer and backpacking enthusiast who hiked in the area three summers ago. "This is country that is really up and down. It's harsh and rugged, with steep terrain, lots of downed logs and thick brush."
DiMaggio is suspected of killing Hannah's mother Christina Anderson, 44, and her 8-year-old brother Ethan Anderson, whose bodies were found Sunday night in DiMaggio's burning house in California near the Mexico border.
Ethan Anderson's remains were not positively identified until Friday night, when the San Diego County Sheriff's Department said its crime lab had used DNA to determine Ethan's identity. An Amber Alert was initially issued for both children.
DiMaggio's car was found Friday morning about 40 miles east of the tiny town of Cascade, parked where the dirt road ends and the Sand Creek trailhead enters the wilderness area. A bomb team determined Friday afternoon the car contained no explosive devices; officers from San Diego planned to search the vehicle for any clues Saturday.
The discovery of the car came about two days after a horseback rider reported seeing the man and girl hiking in the area. Ada County Sheriff's department spokeswoman Andrea Dearden, who is helping the Valley County sheriff's department handle the case, said the rider didn't realize the pair were being sought until he got home and recognized them in news reports.
There have been no other reported sightings of the pair since Wednesday, but the discovery launched a massive search in the southwest corner of The Frank Church-River of No Return Wilderness.
The search area is bisected by the Middle Fork of the Salmon River, a wild waterway that winds through steep canyons and dense forests. The river is extremely popular for recreationists and floaters, some of whom will pay up to $2,000 for multi-day, guided trips down the river.
But away from the river, it's easy to disappear, said Jared Hopkinson, the owner of Rocky Mountain River Tours in Stanley, Idaho.
"If you wanted to go days without being seen, that's the place to do it," said Hopkinson. "There's a few river lodges that are accessible by fixed wing plane and raft, but other than that it is untouched by mankind, the same way it was when there were dinosaurs."
Police have set up checkpoints in the area where the car was found and near other nearby trailheads.
Law enforcement officials in San Diego have noted that DiMaggio bought camping gear a few weeks ago.
DiMaggio was close to the family. Brett Anderson, Hannah and Ethan's father, has described him as a best friend and said his children thought of him as an uncle.
Authorities have said DiMaggio had an "unusual infatuation" with the 16-year-old, although the father said he never saw any strange behavior. If he had, he said, "we would have quashed that relationship in an instant."
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AP writer Elliot Spagat contributed from San Diego. AP writers Bob Jablon and John Antczak contributed to this report from Los Angeles. AP writer Todd Dvorak contributed from Boise.