SAN DIEGO -- An arsonist -- possibly one who travels by motorcycle -- is to blame for a wildfire that has denuded nearly 700 acres of timber terrain in a wilderness preserve near Julian since Tuesday, authorities reported Friday.

Some 1,300 firefighters have worked around the clock for more than 72 hours to surround the blaze on the rugged flanks of 5,300-foot Volcan Mountain, according to the California Department of Forestry.

By Friday afternoon, they had the roughly 685-acre burn area 45 percent surrounded, CDF public information officer Audrey Hagen said.

"The fire is not actively burning at all," she said, "so they're still working on hot spots."

Officials expected cooler weather to help crews fully contain the fire by Sunday evening, though it may take several more weeks to fully extinguish the last smoldering embers.

Authorities declined to disclose why they had ruled the cause as arson, other than to say a motorcyclist was seen leaving the point of origin as the blaze erupted near Farmer and Wynola roads late Tuesday morning.

Since then, the flames have leveled six outbuildings, 28 trailers, three automobiles and a boat, and four firefighters have suffered minor injuries, the CDF reported.

The blaze marched over slopes and through canyons thickly wooded with oaks and pines, many of the trees dead or dying due to bark beetle infestation.

Fortunately for residents of the scenic highland region in eastern San Diego County, the flames consistently moved away from populated areas.

Such was not the case in August 2002, when a law enforcement helicopter clipped power lines and sparked what became known as the Pines Fire.

That conflagration blackened some 62,000 acres and destroyed 37 homes, 116 outbuildings and 169 vehicles.