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POTRERO, Calif. — Ground and airborne crews laboring for a fourth day to subdue a 6,723-acre wildfire that has destroyed 17 homes and outbuildings and forced widespread evacuations in the far southern reaches of San Diego County had the sprawling burn area about 20 percent contained early this evening.

Upward of 1,000 structures in back-country communities near the U.S.- Mexico border remained threatened as the conflagration, dubbed the Border Fire, continued spreading across rugged terrain largely covered with thick chaparral and other tinder-dry foliage, according to Cal Fire.

The blaze erupted for unknown reasons late Sunday morning near state Route 94 and state Route 188 amid an extreme heat wave and soon forced authorities to clear residents out of nearby Potrero.

As the flames rapidly spread north of Tecate Junction on Monday, the evacuation order was expanded to include Canyon City, Cowboy Ranch, Dog Patch, Forest Gate and Star Ranch.

On Tuesday, officials directed people who live and work in Lake Morena Village, a rural community northwest of Campo, to gather what possessions they could carry and flee the advancing fire.

Displaced residents were advised to take refuge in a makeshift shelter at Los Coches Creek Middle School on Dunbar Lane in Alpine. Those with livestock were given the option of taking their animals to a Border Patrol station on Ribbonwood Road in Boulevard. A second large animal shelter at Circle T Ranch in Descanso was about at capacity, according to county officials.

By late Wednesday afternoon, the fire had retreated far enough from Potrero that authorities gave a go-ahead for residents of the rural community to re- enter their neighborhoods, beginning at 4 p.m. The general public was given access to the community as well two hours later, officials said.

The blaze has leveled five homes and a dozen auxiliary buildings, left three firefighters with minor heat-related ailments and forced the closure of the Pacific Crest Trail from the southern boundary of Cleveland National Forest to just south of Hauser Canyon.

Though temperatures have cooled down somewhat over the last two days, steep terrain, extremely dry vegetation due to years of drought, difficult access and low humidity were continuing to produce “erratic fire behavior, thus hampering control efforts,” according to Cal Fire.

State Route 94 was blocked off from state state Route 188 to Mountain Empire RV Park until further notice. Due to the closure, Metropolitan Transit System officials modified Route 894 to only service the area between El Cajon and Tecate.

The blaze also caused power outages that at one point affected more than 1,000 homes and businesses in Potrero and the nearby communities of Campo and Dulzura, according to San Diego Gas & Electric.

About 150 addresses remained without power today, with service expected to be restored to all the affected areas by midday Thursday, SDG&E reported.

About 1,900 firefighters were battling the blaze today on the ground and aboard four air tankers and a half-dozen water-dropping helicopters.