For starters, the search area is roughly 300 square miles of rugged and unforgiving terrain. Much of it is so remote that some ground crews were having to be flown in. Others were on horseback.
The suspect, James Lee DiMaggio, is also presumed to be armed and dangerous, limiting the ability of authorities to use volunteers in the field.
Specialized teams, including some from the FBI, have come in to help with the multiagency search, FBI Special Agent Jason Pack said Saturday.
"In a typical search and rescue, when someone goes missing in the wilderness, you're able to gather large amounts of volunteers to cover the area quickly," he said. "In this particular case, since we presume the suspect to be armed, we have to have the highly trained, enhanced SWAT teams and law enforcement go in."
Pack emphasized that all levels of law enforcement were working together on the case, and federal authorities were relying on the first-hand knowledge of the area provided by state and local agencies.
Speaking Saturday morning outside the fire station in this town of 997, Ada County sheriff's spokeswoman Andrea Dearden said collecting clues from the public and out in the field will be crucial as the search progresses.
The search area, while expansive, is a pinprick of the roughly 2.3 million-acre Frank Church River of No Return Wilderness, but it is rugged, steep and dense.
For the 150 or so searchers who started out on the ground Saturday, that meant no car access.
"We can't go inch-by-inch across this land," Dearden said. "We're going to have to really focus on whatever evidentiary clues that we can find. That's what we're going to be looking for. That's why we want people to call us. We want tips. We need to know if anyone sees anything that is out of the ordinary."
Officials hoped to have 200 searchers on the ground by day's end, she added.
Dogs are also on hand as needed and air crews would be assisting from above.
The nearly weeklong search for DiMaggio, 40, and 16-year-old Hannah has centered on Idaho after a major break in the case Friday. DiMaggio's blue Nissan Versa — the subject of a five-state Amber Alert — was found roughly 60 miles northeast of Cascade, at a trail head leading into the wilderness.
It was "the last place you can go before you hit the wilderness and stop driving," Dearden said.
The search of the area began after what authorities initially described as a group of horseback riders reported seeing a pair thought to be Hannah and DiMaggio on Wednesday morning.
The Ada County Sheriff's Department on Saturday issued a statement saying it was a solo rider who encountered the pair, not realizing until later that day that the girl may have been Hannah.
The witness said the man and girl had a tent and backpacks but looked out of place in the rugged terrain because they had light camping equipment, authorities said. The witness also told authorities the girl appeared to be well.
The car, covered in brush and missing its license plates, was being scoured for evidence Saturday, Dearden said. Authorities thought DiMaggio might have rigged it with explosives, but a bomb squad determined nothing was inside.
The search for the girl began Sunday night after the bodies of her mother, Christina Anderson, and a boy later determined to be her 8-year-old brother Ethan were found at DiMaggio's burning home in eastern San Diego County.
Local, state and federal authorities were involved in the Idaho search, and were in close communication with investigators in San Diego County.
"We're a team. One team, one fight is our mantra," Pack, of the FBI, told reporters. "We know that the state and locals know the area better than we do .... They hike here, they fish here. We know that they have the best knowledge of the area."
The goal, he added, is to find Hannah.
Authorities planned to increase the number of search personnel throughout the day.
"Our focus is on the safe recovery of the young lady,” Pack said. "We're hoping to do everything we can to find them as quickly as possible."