SAN DIEGO COUNTY -- Downed power lines and a quick-spreading structure blaze are to blame for the devastating fires in Fallbrook and Pauma Valley, respectively, but the cause of the fires raging through the rest of the region have not yet been determined, authorities said Thursday.

No further details were immediately available on the power lines that sparked the Rice fire, which started in the Fallbrook-Rice Canyon area on Monday and had burned 9,000 acres by Thursday evening, said Roxanne Provaznik, spokeswoman for the California Department of Forestry and Fire Protection.

In Pauma Valley, the 35,000-acre Poomacha fire started Tuesday when a structure fire raged out of control on the La Jolla Indian Reservation, fire officials said. That blaze prompted evacuations in Valley Center and was 20 percent contained as of Thursday.

It was not known Thursday whether arson was involved in any of the other fires, including the county's two largest blazes -- the 200,000-acre Witch Creek fire and the 81,000-acre Harris fire -- or the 10,000-acre Horno/Ammo fire on Camp Pendleton, Provaznik said.

"I talked to the investigator today, and there are several more people to be interviewed," Provaznik said.

Those three blazes, which span from the inland hills of Ramona to the coastal Marine base, have leveled at least 845 homes and resulted in at least seven deaths directly related to the fires.

Arson has been determined as the cause of several other fires, including a 200-square-foot blaze on Calera Street in Vista on Tuesday, sheriff's officials said. Deputies said they arrested Gorgonio Nava, 19, and a juvenile on suspicion of setting the fire, which was doused by firefighters before it spread to nearby structures.

Authorities also blame arson for the 23,000-acre Santiago fire in Orange County, as well as a small brush fire in San Bernardino County that led to Wednesday's arrest of John Alfred Rund of Hesperia. Also in San Bernardino, police shot and killed an arson suspect who allegedly backed his car into a patrol cruiser after a chase on Tuesday.

The Associated Press contributed to this report. Contact staff writer Sarah Wilkins at (760) 740-3524 or swilkins@nctimes.com.