A helicopter battling one of the nation’s largest active wildfires crashed while navigating treacherous terrain in central Utah on Friday, killing the two firefighters who were on board, according to federal officials.

The aircraft, one of several fighting the fast-spreading Widemouth 2 blaze burning through Fishlake National Forest and the surrounding communities, went down near the city of Richfield around 10 a.m. The wreck sparked another fire, officials said, and flames quickly overran the area, initially preventing emergency personnel from reaching the site.

“No escape routes that any firefighters could get into or out of if we had put them in there,” said Nathan Curtis, the sheriff of Sevier County, where the crash occurred.

Overnight, the authorities remained uncertain about the fate of the pilot and crew member aboard the helicopter, which had been working on contract with the U.S. Forest Service, but by Saturday morning their worst fears had been confirmed.

The National Interagency Fire Center, a coalition of federal and state agencies, reported their deaths in its daily bulletin.

Officials have not identified the firefighters, and they were still working to determine the cause of the crash. Investigators with the National Transportation Safety Board traveled to the area but were waiting for fire conditions to improve before they could reach the crash site, said a spokeswoman for the agency, Sarah Taylor Sulick.

In an unsigned statement, the Forest Service said only that it was “an emerging situation” and that “safety actions are prioritized first.”

But even as many central questions remained unanswered, Friday’s fatalities deepened an already solemn mood that has taken hold at base camps and incident command centers across the West this year, as a raging wildfire season has claimed the lives of several other first responders. In June, three wildland firefighters died while battling a blaze along the Utah-Colorado border, and on Thursday a bulldozer operator was killed in a southern Oregon fire.

“Wildland firefighting is among the most dangerous jobs in the world,” Gov. Spencer Cox of Utah said in a statement on Friday, responding to the crash in his state. “Today is a painful reminder of the extraordinary risks these brave men and women accept to protect us.”

Authorities believe the Widemouth 2 fire began with a lightning strike on July 27 and quickly scorched more than 100,000 acres, feasting on tinder-dry forests and propelled by unpredictable winds. It is the latest large blaze in what researchers say is one of the state’s worst fire seasons on record, with Utah — and the rest of the West — struggling through a brutal drought and sweltering summer.

“This is our third fire to be right at 100,000 acres this year, which is not a record that we’re proud of,” Mr. Cox told reporters during a tour of the fire line on Thursday. He said the fierce fire season had exhausted emergency workers and spread resources thin.

When the firefighting helicopter, a Sikorsky S64, took off on Friday morning, conditions were safe enough to fly, Tyler Hecht, a deputy commander with the Great Basin Incident Management Team, said at a news briefing later that day. The terrain was steep and tricky, Mr. Hecht said, but it was not unusual for this type of wildfire.

As Widemouth 2 continued to rage, with just about a quarter of the blaze contained, officials were left hoping for rain — just not too much, as communities near fire burn scars are now fearing flash floods and mudslides.

“Really,” Mr. Cox said, “we need some help from Mother Nature.”

Madaleine Rubin contributed reporting.