By early Friday evening, the fire had reached 3,100 acres and was 20 percent contained, CAL FIRE said.

The blaze, which has caused no known structural damage or injuries, erupted for unknown reasons in the Cigarette Hills area of eastern San Diego County, near Banner Grade, early Thursday afternoon. Within two hours, it had grown to more than 1,000 acres, driven by steady winds out of the west.

Shortly after the blaze started spreading, authorities cleared residents out of about 100 back-country residences potentially in its path in Shelter Valley. Those evacuees were allowed to return to their homes late Thursday night. The fire also forced closures of stretches of state Route 78 and county Route S2 at times. By late this morning, crews were making solid headway against the flames, according to Cal Fire Battalion Chief Nick Schuler. "(There was) a lot of work done overnight," Schuler said. "The winds have increased a little bit, but we're making good progress."

Full containment of the fire is expected May 30th.

It was unclear what sparked the flames, which moved through part of an area scorched last October by a 2,100-acre blaze dubbed the Great Fire.