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.
Manhunt Covers Western States and MexicoEvidence 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.
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.
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. Brett Anderson, the children's father and Christina's husband, recently moved to Tennessee.