Tens of thousands of homes and buildings were threatened Sunday by an out-of-control wildfire burning in the foothills of a national forest east of Los Angeles, amid a days-long heat wave that pushed temperatures into the triple digits across the region.

State firefighters said three firefighters had been injured and more than 35,000 structures were threatened, including single and multi-family homes and commercial buildings, while authorities issued evacuation orders for several areas. Thunderstorms expected later in the day could make conditions even more challenging.

The so-called Line Fire was burning along the edge of the San Bernardino National Forest, about 65 miles (105 kilometers) east of LA. As of Sunday morning, the blaze had charred about 27 square miles (70 square kilometers) of grass and chaparral, leaving a thick cloud of dark smoke blanketing the area.