Boise, Idaho ? Two firefighters were overrun by flames and killed soon after they were dropped by helicopter to battle a fast-moving blaze in a national forest in central Idaho, officials said Wednesday.

The fire in the Salmon-Challis National Forest about 130 miles south of Missoula, Mont., was caused by lightning and first reported Sunday night. Hot temperatures and wind blew it up from 120 acres to about 1,000 acres Tuesday night.

Killed were Jeff Allen, 23, of Salmon, and Shane Heath, 22, of Boise.

U.S. Forest Service spokeswoman Erin O’Connor said the deaths were under investigation.

Allen and Heath were the eighth and ninth firefighters to die since February nationwide, said Alan Hoffmeister, a spokesman for the National Interagency Fire Center in Boise. Only one of the previous deaths was by fire; the others were in helicopter crashes and accidents.