Firefighters struggled early Wednesday to prevent a fire that has charred nearly 39 square miles of brushland from crossing Interstate 5, which would force authorities to again shut down traffic on one of California's busiest freeways.

The fire, which was sparked on Labor Day by someone burning debris in a remote area, has burned 24,943 acres and was 25 percent contained, said Ed Linquist, a spokesman for the U.S. Forest Service. It was burning in the Los Padres National Forest and the adjoining Angeles National Forest.

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Winds were pushing the fire, which was growing slowly to the east and the southwest. Spots of flames jumped over a fireline cut by a bulldozer next to the highway, and erratic winds were spreading the fire through embers, said Kathy Good of the U.S. Forest Service.

Flames and smoke intensified along the interstate Tuesday, prompting officials to close a stretch for the second time in as many days. Authorities reopened two northbound and two southbound lanes by early Wednesday, advising motorists to drive cautiously.

No one has been injured and no major structures have burned, though fire officials were concerned the blaze could advance toward Paradise Ranch, a mobile home park a few miles south of Pyramid Lake. Also threatened was a condor sanctuary in the Sespe Wilderness, but so far firefighters have not spotted any condors, Linquist said.

"They may be roosting elsewhere at this time," he said.

About 1,500 firefighters and a dozen helicopters and airplanes were working to draw a 26-mile containment line around the fire, which was burning on chaparral-covered ridges.

Firefighting costs had risen to $7.4 million.