A wildfire in the Rio Grande Valley spread to 50,000 acres early Wednesday, forcing hundreds from their homes as officials pinpointed a fallen power line as the source of the massive blaze.

At least four homes were destroyed, and bulldozers uprooted grass near the Hidalgo County Jail and an elementary school in hopes of stopping the encroaching flames, authorities said at an early Wednesday news conference.

At least 500 people were evacuated but no injuries had been reported, The Monitor in McAllen reported.

"Right now, our priority is to save lives," Hidalgo County Judge J.D. Salinas said.

The fire that started in Brooks County around 4 p.m. Tuesday had crept into Hidalgo and Willacy counties, officials said. Salinas said a nearly 40-mile stretch of U.S. Highway 281 -- the main route north out of area -- would be closed for the immediate future.

Firefighters estimated the blaze as 18 miles wide with flames leaping 40 feet into the air, the newspaper reported. Winds upward of 30 mph were expected to continue blowing until around dawn Wednesday, officials said, hampering efforts by firefighters to contain the blaze.

Deputies had gone door-to-door through the area Tuesday night to alert residents to the threat. Edinburg North High School was opened as a shelter, Hidalgo County spokeswoman Cari Lambrecht said.

The wildfire began after President Bush made federal aid available to 70 percent of Texas counties last weekend after wildfires scorched 133,000 acres across the state Friday.