LITTLETON, Colo. (AP) — A wildfire burning in Colorado’s heavily populated Front Range region has burned dozens of homes and outbuildings, while a second fire crept within a quarter-mile of evacuated homes near Denver on Thursday.

Authorities said they were hopeful that hundreds of threatened homes could be saved. But firefighters working in the tree-covered foothills on the eastern edge of the Rocky Mountains faced sweltering temperatures, and some were sidelined by heat exhaustion.

The struggle to contain the blazes came after authorities said Wednesday that a person was killed in a wildfire west of Lyons, Colorado. The person’s remains were found inside one of five houses that burned.

Almost 100 large fires are raging across the West. The largest, in Northern California, has burned more than 400 houses and other structures, officials reported.

New, large fires were reported in Idaho, southeast Montana and north Texas.

About two dozen homes and outbuildings were damaged or destroyed in a wildfire near Loveland, Colorado, authorities disclosed after an initial survey of the burn area.

Meanwhile, the Quarry Fire southwest of the Denver suburb of Littleton encroached on several large subdivisions after people in 600 homes were ordered to evacuate.

By Thursday afternoon, firefighting aircraft zipped back and forth between the blaze and a nearby reservoir. Planes skimmed the surface to scoop up water and hovering helicopters pumped water into their tanks before returning to the fire to dump their loads.

Jim and Meg Lutes watched from an overlook near their house northeast of the fire as smoke plumed up from the ridges. Their community west of Littleton was not under evacuation orders, but the couple had been ready to start packing a day earlier when flames could be seen blanketing the mountains.

“It can come over that hill pretty quick if the wind changes,” said Jim Lutes, 64, pointing to a nearby ridge.

Five firefighters were injured Wednesday, including four who had heat exhaustion, said Mark Techmeyer, a spokesperson with the Jefferson County Sheriff’s Office.

The fire was in steep terrain that made it difficult to access but had been held to less than one square mile (1.8 square kilometers) with no houses yet destroyed, authorities said. It remained a major hazard, with hot temperatures and low humidity elevating the danger.

The fire was discovered Tuesday night by a deputy on patrol in what authorities described as “open areas.” It was small at first but quickly spread.

“I think anytime that you see a small fire in an area like that at 9:00 at night, it makes you scratch your head a little bit,” Techmeyer said at a Wednesday briefing when asked if he thought the fire was suspicious.

On Thursday he said the fire has “proven to be one of the most challenging firefights I’ve seen,” as helicopters passed overhead.

He was flanked by hillsides where firefighters labored to keep the blaze from hopping a road separating the conflagration from populated areas.

“If we lose that fight, the fire is coming this way,” Techmeyer said, motioning toward dense neighborhoods and Littleton.

The ranks of firefighters more than doubled since the day before, from 75 to 155, most of them volunteers. No structures had been lost as of Thursday afternoon but authorities expected the battle to be a long one.

Defense contractor Lockheed Martin, which operates a large facility just outside the evacuation zone, closed it as a precaution.

Miles to the north near Lyons, officials lifted some evacuations and reported making progress on the Stone Canyon Fire, which has killed one person and destroyed five houses. The cause is under investigation.

California’s arson-caused Park Fire northeast of Chico continued to grow, covering about 610 square miles (1,590 square kilometers) as of Thursday morning. That’s more than 25 times the size of New York’s Manhattan Island.

Losses also increased. The latest updates tallied 437 structures destroyed and 42 damaged, according to Cal Fire. The fire was 18% contained.

Authorities said they faced critical fire weather in the coming days with potential triple-digit temperatures, thunderstorms and erratic winds. Almost 6,000 personnel were helping battle the Park Fire as more fire crews arrived from Utah and Texas.

Scientists say extreme wildfires are becoming more common and destructive in the U.S. West and others parts of the world as climate change warms the planet and droughts become more severe.

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Brown reported from Billings, Montana.