(New throughout, adds details on abduction, quotes from authorities)
By Marty Graham
SAN DIEGO, Aug 9 (Reuters) - Horseback riders returning from the remote Idaho wilderness have told police they may have encountered a California man suspected of killing a longtime friend and kidnapping at least one of her children, the San Diego County sheriff said on Friday.
The riders told police that on Wednesday they came across fugitive James Lee DiMaggio backpacking in the Frank Church River of No Return Wilderness northeast of Boise with a teenage girl who might have been the 16-year-old he is accused of abducting, Sheriff William Gore said.
The group said they spoke briefly with the man and his young companion and realized only after returning from their excursion that night and seeing news reports that they may have encountered DiMaggio, 40, and his alleged victim.
The horseback riders said the two appeared to be in good health, and there was no indication the girl was in distress or being held against her will.
"They did seem to think the two of them were out of place in that area, with the light camping equipment they had," Gore said. "It is very ... rugged terrain."
Police acting on the sighting on Thursday morning found a blue Nissan Versa belonging to DiMaggio near a trailhead about five miles away, covered in brush and with the license plates removed, Gore said.
DiMaggio is wanted in the killing of Christina Anderson, 44, and in the disappearance of her two children, 16-year-old Hannah and 8-year-old Ethan, all of whom were last seen on Saturday, the day before he is suspected of setting his own house on fire.
Since then, DiMaggio has been the subject of a massive manhunt. Authorities have issued child-abduction alerts across the U.S. West and notified authorities in Canada and Mexico to watch for the fugitive and either of the Anderson children.
San Diego County Sheriff's Captain Duncan Fraser said that the camping gear being used by DiMaggio and the girl had been purchased several weeks earlier.
"We believe this was a planned event. The information we're working with now tells us that this was planned," Fraser said.
The computer technician has been described as a longtime friend of the Anderson family who was like an uncle to the children.
The Sheriff's Department in Ada County, Idaho, said investigators from multiple agencies were en route to the mountainous area where the horseback riders believe they came across DiMaggio and Hannah Anderson.
Sheriff's officials declined to say if explosives were found rigged to the car, as police had cautioned they might be.
"This is a very remote section of land that is not easily accessible. Because of that, gathering credible information from the scene will take some time - likely several hours," the Ada County Sheriff's Department said in a statement.
"Teams are stationed at multiple access points around the wilderness area to monitor any people coming in or out. At this time, the location of Hannah Anderson, Ethan Anderson and James DiMaggio is not known," the sheriff's office said.
Two people trying to pack into the Idaho backcountry with light camping gear and supplies could be ill-equipped to master the challenges of the intensely rugged terrain, said Chris Grove, district ranger with the Salmon-Challis National Forest, which oversees the Frank Church River of No Return Wilderness.
Nighttime temperatures in the high country are already dropping below freezing and cross-country travelers will face numerous obstacles on trails like large, fallen trees and steep grades that may lead to nowhere, Grove said.
"The path you may choose may take you to a cliff or a river like the Middle Fork that is difficult to cross," he said of an area known for forested peaks, sheer canyons and whitewater rapids.
In California, police have not revealed how Christina Anderson was killed. They have said a child's body found in DiMaggio's burned-out home in the San Diego County community of Boulevard may be that of Ethan Anderson, but that more time was required for a positive identification.
The children's father, Brett Anderson, who now lives in Tennessee, has said he presumes the body is that of his son. He pleaded with DiMaggio in a televised message this week to let his daughter go and advised the teenager to make a run for it.
Authorities have said they have no evidence of a precipitating incident or circumstances that might have led to the crimes DiMaggio is suspected of committing. (Additional reporting by Laura Zuckerman in Idaho and Steve Gorman and Dan Whitcomb in Los Angeles; Writing by Dan Whitcomb; Editing by Cynthia Johnston, Philip Barbara and Chris Reese)