Police got their first big break on Wednesday, after a man riding his horse through the rustic Frank Church – River of No Return Wilderness Area chatted along a trail with a man and a teenage girl with hiking and camping gear.
The three "exchanged pleasantries," and the man and girl indicated they were on a hiking trip, Andrea Dearden, a spokeswoman for the Ada County Sheriff's Department, tells PEOPLE.
The rider sensed nothing suspicious about the encounter until he got home later that day and saw news reports about the search for James DiMaggio, 40, and 16-year-old Hannah Anderson, Dearden says.
"There was nothing that caused the horseback rider any alarm," Dearden tells PEOPLE. "But after he saw the pictures, he thought it might have been them."
The report reinforces what San Diego County authorities already suspected: that Hannah's younger brother, Ethan – whom they feared was killed by DiMaggio when he also allegedly killed the children's mother and dog and burned his own cabin down – was not alive and with DiMaggio.
Idaho state police investigating the sighting got the next big break Friday morning, when they found DiMaggio's blue Versa near a trailhead, covered with brush, roughly six miles from where the horseback rider recalled seeing DiMaggio and Anderson. The license plate had been removed, but the VIN was a match, officials said.
The vehicle's discovery prompted an impromptu press conference Friday where San Diego County Sheriff Bill Gore said he was hopeful that Hannah will be returned safely to her family. "As far as we know, it didn't appear [Hannah] was being held against her will," Gore said.
Dearden said local, state and federal authorities are searching the rugged, mountain terrain, which spreads across 2.3 million acres, and have blocked all of the trails leading away from the area where the pair was spotted.
"We're using as many resources as we possibly can to try to locate them," Dearden tells PEOPLE.
DiMaggio is wanted on suspicion of murder and arson in a search that began in California and spread to Oregon, Washington, Nevada, British Columbia, and Mexico's Baja California state before reaching Idaho. A motive for the murder-abduction is unclear, although multiple friends of Hannah's tell PEOPLE that DiMaggio recently confessed to her that he had a crush on her.
With Reporting by KEN LEE