As wildfires raged across the western US, authorities said on Wednesday that human remains had been found inside a burned house in Colorado. There are nearly 100 active wildfires burning in the west, including a massive wildfire in California that has grown swiftly in recent days to become the fifth-largest in state history. In Colorado, a wildfire near the city of Loveland in the Rockies grew to more than 5,000 acres (2,023 hectares) on Tuesday evening as more homes were placed under mandatory evacuation orders and a looming column of smoke could be seen for miles around. A body was discovered in a home that burned about 1 mile (1.6km) north of Lyons, Colorado, according to Curtis Johnson, the Boulder county sheriff. He said detectives were assisting the investigation into the death but declined to provide further details. The fatality is one of several reported in recent weeks as the US wildfire season kicked into high gear. An air tanker pilot in Oregon was recently killed after his plane crashed while he was fighting a fire, and human remains were found in a burned-out house in Mendocino county, California, in mid July. Meanwhile, firefighters are battling to hold back California’s largest wildfire of the season. Authorities said the Park fire in northern California is continuing to rage at intense speeds in rugged terrain outside the university town of Chico, and is threatening to spread to two other counties as temperatures are set to warm later this week. Fire crews managed to reach 18% containment in recent days, and evacuation orders in some communities have been lifted. Cooler weather has helped firefighters stop the blaze’s path near some communities such as Forest Ranch, where some people began returning to unscathed homes on Tuesday. Christopher and Anita Angeloni have lived in the community of 1,600 for 23 years and have had to evacuate several times due to wildfires, they said, including during the 2018 Camp fire that killed 85 people and decimated the town of Paradise, about 8 miles (13km) south. Christopher Angeloni said he constantly worked on creating defensible space around his home and was happy to return home nearly a week after evacuating to see his hard work had paid off. “We were prepared to possibly lose everything,” he said. Anita Angeloni said it had been a stressful week. “We have not been sleeping enough, very tense, praying all the time, teary eyes,” she said. “But we’re here. We’ll see for how long.” The fire had scorched 386,764 acres by late Tuesday evening, according to Cal Fire, the state’s fire agency. More than 270 structures have been destroyed, and about 8,200 remain threatened. The fire started last Wednesday after authorities say a man pushed a burning car down a ravine in Chico. The suspect, Ronnie Dean Stout II, was charged with arson on Monday. His public defender, Nicole Diamond, said in an email she had no comment. Meanwhile in southern California, people in Kern and Tulare counties were ordered to evacuate because of a fire sweeping through the Sequoia national forest. That fire, which has been dubbed the Borel fire, scorched through almost the entirety of the historic mining town of Havilah, officials said. The fires burning throughout California have overwhelmed the state’s firefighting capacity and outside help has started to arrive, officials said. The California governor, Gavin Newsom, thanked his Texas counterpart, Gregg Abbott, on Tuesday for sending more than two dozen fire engines to help combat the Park fire this week. It has been a brutal wildfire season in the US west this year, with more than 100 large active wildfires burning across the country. Typically, the most intense period in the US wildfire season comes in late August. In Oregon, wildfires have burnt more than 1m acres in recent weeks. The largest of them, the Durkee fire, has scorched nearly 285,000 acres. Firefighters there said cooler weather had allowed them to make significant progress in battling the conflagrations. Some of the worst fires have sent smoke billowing for hundreds of miles around them, blanketing cities like Boise, Idaho, and Calgary, Canada, with poor-quality air. Wildfire smoke is known to have strong adverse health effects. Smoky skies have been linked to spikes in hospital admissions and ambulance calls for conditions such as asthma. For people who have cardiovascular problems, the risk of cardiac arrest rises 70% during days with heavy smoke. A recent study in California attributed more than 50,000 premature deaths to wildfire smoke exposure. Western states have spent weeks in the grip of back-to-back heatwaves, some with record-breaking triple-digit temperatures, compounding the risk of new ignitions as a parched landscape is primed to burn. The conditions were ripe for a destructive fire, said Don Hankins, a pyrogeographer and Plains Miwok fire expert at California State University, Chico, about the lead-up to the Park fire. “We’ve got a south wind that’s carrying this and hot dry conditions combined with the amount of fuels [the area] accumulated,” said Hankins, who was evacuated because of the blaze for days. “A lot of those areas started to dry out early in the season. It’s just a recipe for once you have an ignition, a human caused-ignition, things aligned to create this situation.” The region has experienced too many of these kinds of disasters in short succession, Hankins said, and far more prescribed fire is needed to reduce the accumulation of fuels that leads to more destructive blazes. “Each one of these fires provides a lesson. Ultimately the lesson is we need to be stewarding our land,” Hankins said. “There is no choice of not having fire in this landscape.” Fire crews are bracing for warm temperatures set to return to California later this week. “Above average” temperatures in many areas, in combination with possible lightning outbreaks and occasionally breezy winds, may spell trouble for firefighters, said the climate scientist Daniel Swain on X.