The victims' identities haven't been released, and the boy's motive and connection to the five victims weren't immediately known. Williamson said investigators were trying to determine if the victims were related.
"We are trying to identify the victims," Williamson said.
Each victim suffered more than one gunshot wound, he said.
Investigators also were trying to determine who owned several guns that were found at the home, one of which was a semi-automatic military-style rifle.
Authorities declined to release details of any conversation that the 15-year-old had with investigators.
The teenager was booked on two counts of murder and three counts of child abuse resulting in death.
On Sunday, a police roadblock cut off public access to the narrow dirt road that leads to the home.
Detectives were still processing the crime scene Sunday afternoon.
Peter Gomez, a 54-year-old carpenter who lives about 200 yards from the home, said he saw police vehicles and ambulances arrive Saturday night.
Gomez said he doesn't know the family that lives at the house, but he has seen a married couple and their two boys and two girls from time to time.
"It's a horrible thing," Gomez said. "You see all this stuff that happens all over the country, the shootings in the schools and theaters, and then it happens right here. It's sad."