James Lee DiMaggio, 40, may have had an "unusual infatuation" with the missing girl, Hannah Anderson, said San Diego County Sheriff's Capt. Duncan Fraser.
"That is kind of a working theory, that it may be something of a motivator," Fraser said Thursday.
On Sunday night, authorities found the body of Hannah's mother -- 44-year-old Christina Anderson -- when they extinguished flames at DiMaggio's rural home. A child's body also was discovered as they sifted through rubble in Boulevard, a tiny town 65 miles east of San Diego.
The body may be that of Hannah's 8-year-old brother, Ethan. Fraser said it could take several days to identify the badly burned remains. Investigators were unable to extract DNA.
Evidence found in the rubble suggested DiMaggio may have fled with homemade explosives, Fraser said, declining to elaborate on what was discovered. The car may be booby-trapped, he said.
"In the event that someone comes across the car, they need to use caution," Fraser said.
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. An additional eight FBI agents were assigned to a command post at San Diego sheriff's headquarters, as state and local law enforcement agencies were on alert.
Oregon State Police fielded more than 130 tips after authorities issued an Amber Alert for DiMaggio and his blue Nissan Versa with California license plates. A possible sighting was reported in northeast California near Alturas Wednesday afternoon, followed by another about 50 miles along the same highway near Lakeview, in south-central Oregon.
Fraser, whose office had hundreds of leads on DiMaggio's whereabouts, said the Oregon tip appeared "very credible"
"We're taking it very seriously," he said.
Washington State Patrol responded to reports of sightings on Interstate 5, including the Tacoma area, Vancouver and at least one near Bremerton, said Sgt. Jason Hicks.
Messages seeking DiMaggio and the Anderson children appeared on digital billboards and freeway signs, said Bob Hoever, director of special programs for the National Center for Missing & Exploited Children. Loud tones beeped on mobile phones in the four western states where Amber Alerts were issued.
In Mexico, airports, bus and taxi companies and law enforcement agencies were on the lookout, said Alfredo Arenas, international liaison for the Baja California state police.
"This is a pretty much an all-hands-on-deck effort. It's huge," Fraser said.
DiMaggio, a telecommunications technician at The Scripps Research Institute in San Diego, was like an uncle to Hannah and Ethan Anderson. He was close to their parents for years.
Christina Anderson's father, Christopher Saincome, said his daughter visited DiMaggio's home to say goodbye before he moved to Texas.
Dawn MacNabb, whose son, Alan, was close friends with Hannah, said Hannah told her son Friday that the Andersons were going to visit DiMaggio at his house before he moved.
"She told him Jim was depressed, that it was his last weekend," MacNabb said.
DiMaggio told Hannah Anderson a couple months ago he had a crush on her and would date her if they were the same age, said Marissa Chavez, 15, a friend who witnessed the remarks when DiMaggio was driving them home from a gymnastics competition. Chavez said Hannah was "a little creeped out by it."
DiMaggio argued with Hannah when he took her alone to Hollywood to celebrate her 16th birthday last month, Chavez and MacNabb said.
MacNabb said DiMaggio occasionally took Hannah and her friends to his house and that she felt uncomfortable enough to warn Christina Anderson.
"She really trusted him," MacNabb said. "I have been concerned for a while. It's not normal behavior."
Brett Anderson, the children's father and Christina's husband, recently moved to Tennessee.
Take ABC13 with you!
Download our free apps for iPhone, iPad, and Android devices