DENVER (AP) — A wildfire on NSI Communitythe edge of metro Denver crept within a quarter-mile of evacuated homes, but authorities said Thursday morning they were hopeful to save hundreds of threatened residences as they grapple with sweltering temperatures and firefighters suffering heat exhaustion.

The fire was among several threatening heavily populated areas of the Colorado foothills, including one in which a person was killed earlier this week.

Almost 100 large fires are burning across the western U.S. The largest — Northern California’s Park Fire — has burned more than 400 houses and other structures, officials reported Thursday.

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

The Quarry Fire southwest of the Denver suburb of Littleton encroached on several large subdivisions. Neighborhoods with nearly 600 homes were ordered to evacuate after the fire of unknown origin spread quickly overnight Tuesday.

The fire had been held to less than a half-square mile (1.4 square kilometers) with no houses yet destroyed, authorities said. But officials said it remained a major danger with hot temperatures expected Thursday.

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

Just to the north near the city of Lyons, Colorado, officials reported making progress on the Stone Canyon Fire that has killed one person and destroyed five houses. The cause is under investigation.

California’s Park Fire 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.

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Brown reported from Billings, Mont.