An explosive wildfire in Washington's North Cascades killed four firefighters Tuesday and left another in serious condition.

The U.S. Forest Service said the blaze was apparently caused by an unattended campfire.

The four who died were all from Washington: 40-year-old Tom Craven of Ellensburg, 18-year-old Karen Fitzpatrick of Yakima, 21-year-old Devin Weaver of Yakima and 19-year-old Jessica Johnson of Yakima.

They had been missing since an explosive burst of flames overran a group of 40 firefighters Tuesday evening, Forest Service spokeswoman Debbie Kelly said. Most were able to deploy foil-like emergency fire shelters or otherwise escape. But the four firefighters of a five-member crew were trapped and killed, despite using the shelters.

The seriously injured firefighter, a 21-year-old from Yakima, was flown to a Seattle hospital with burns over 30 percent of his body and was in serious condition early Wednesday. Three other firefighters were treated for lesser burns at other hospitals. Two civilians were also injured, but their condition was not immediately known.

Extremely dry weather, low humidity and dry underbrush made conditions in the steep, heavily forested terrain of north-central Washington especially dangerous. She said the crews thought they had the situation under control until the wind picked up and the fire began spreading quickly.

Stoked by high temperatures and strong wind gusts, the blaze grew from less than 10 acres early Tuesday to 2,500 acres by late afternoon, Kelly said.

After radio contact with two firefighters was lost, a search party set out to locate the missing people and came upon the four victims.

A national investigative team was flying to the scene.

And 600 firefighters were to arrive Wednesday with a special management team to continue battling the fire.

The wildfire near Twisp was one of at least three burning on the eastern side of the Cascade Range. Another wildfire burning about 10 miles to the south, near Carlton, grew to 1,200 acres Tuesday night, officials said. A third fire had burned at least 70 acres near Grand Coulee Dam.

The Associated Press contributed to this report.