The blaze, ignited by lightning and stoked by strong, dry winds and a heat wave that has baked the region in triple-digit temperatures, has charred more than 405 hectares of tinder-dry chaparral and grasslands, fire officials said.

Local television news footage showed an unbroken line of flames stretching along a ridgeline, sending gray brown smoke billowing into the evening sky.

Authorities ordered the evacuation of Yarnell and the adjoining town of Peeples Valley, alerting residents through reverse 911 emergency calls to homes and sending sheriff’s deputies door to door, according to the InciWeb fire news site of the U.S. Forest Service.

The two towns, which lie southwest of Prescott, Arizona, are home to roughly 1,000 people.

Steve Skurja, spokesman for Yavapai County Sheriff’s Office, said at least 200 structures had been destroyed, most of them in Yarnell, a community consisting largely of retirees. Fire officials said most of the buildings lost were homes.

The Daily Courier Prescott newspaper said the dead were members of the Prescott Fire Department’s Granite Mountain Hotshots team.

“We’re an organization and a city that’s in grief,” Fire Chief Fraijo told reporters late on Sunday. “We grieve for the families, we grieve for the department and we grieve for the city.”

The men’s families “are in terrible shock,” he added.

“This is as dark a day as I can remember,” Governor Jan Brewer said in a statement. “It may be days or longer before an investigation reveals how this tragedy occurred, but the essence we already know in our hearts: fighting fires is dangerous work.”

The Yarnell Hills fire was one of dozens of wildfires in several western U.S. states in recent weeks. Experts have said the current fire season could be one of the worst on record.

The National Fire Protection Association lists just seven previous incidents in the United States during the past century that killed as many or more firefighters than on Sunday in Arizona, topped by the loss of 340 firefighters in the Sept. 11, 2001, attack on the World Trade Center.