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 heatwave 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 (105km) east of LA. As of Sunday morning, the blaze had charred about 27 sq miles (70 sq km) of grass and chaparral, leaving a thick cloud of dark smoke blanketing the area. “Afternoon thunderstorms could cause new ignitions and potentially influence activity around the fire perimeter,” state firefighters said in a Sunday morning update. “Hot and dry conditions mixed with thunderstorms are expected to challenge firefighters for the next few days.” County officials, who declared an emergency Saturday evening, issued evacuation orders for Running Springs, Arrowbear Lake, areas east of Highway 330 and other regions. “Extreme temperatures, wind and lightning strikes have allowed the fire to grow rapidly,” the county said in a statement. California governor Gavin Newsom proclaimed a state of emergency for San Bernardino county Saturday night. More firefighters were expected to arrive Sunday. State officials said vegetation is critically dry in the area and temperatures reached more than 100F (39C) on Saturday with relative humidity dipping to provide ideal conditions for the fire to spread. The fire began Thursday evening, and the cause is under investigation. More than 600 firefighters were battling the blaze, supported by water-dropping helicopters that hovered over homes and hillsides, along with other aircraft, according to state firefighters. Fire officials didn’t immediately return a call on Sunday seeking more information about the three injured firefighters. The fire produced coiling clouds of dense smoke, and flames could be seen cresting hillside ridges. No homes or other structures had been damaged or destroyed. The National Weather Service said downtown Los Angeles hit a high of 112F (44C) Friday, which marked the third time since 1877 that a temperature that high has been reached there. Experts have attributed the increasing intensity of wildfires in recent years in part to a landscape that has grown hotter and drier amid the ongoing climate crisis, which is primarily driven by the burning of fossil fuels.