BOISE — Firefighters on Sunday faced the possibility of a long battle against a wildfire burning in heavy timber 50 miles east of Boise after an intense effort a day earlier that included dropping more than 100,000 gallons of fire retardant failed to keep the blaze in check.

Fire spokesman Dave Olson said the 1-square-mile Trinity Ridge Fire in the Boise National Forest expanded Saturday despite 46 runs by four military C-130 cargo planes based at the nearby Boise Air Terminal and more drops by three single-engine air tankers.

On Sunday afternoon, he said retardant drops continued but not at the same pace as Saturday.

"We were really trying to catch this," he said. "Now it has the potential to burn for a longer period of time. We're just worried that there are going to be new starts with any kind of lightning events, and the fewer fires we have to deal with the better."

Olson said the fire started Friday and is burning sub-alpine fir and lodgepole pine that are prone to torching and throwing embers, starting new fires. He said that's what happened Saturday afternoon and thwarted efforts to contain the fire before it got too big.

"The trees will just totally torch out and become fully engaged," he said. "We know that it's an extremely dry forest in that area. We have a type of fuel that is very difficult to build a fire line around. It has a lot of country to burn in. So it will be a challenge."

The fire is burning about 10 miles northwest of Featherville in Elmore County. Olson said no structures are threatened, but the area includes habitat for bull trout, a threatened species. More firefighters are being called in to bolster the 130 at the scene and a Type 2 Incident Management Team was expected to arrive later Sunday. Some parts of the Boise National Forest near the fire have been closed.

Olson said the fire is likely human caused because firefighters found a burned utility terrain vehicle at the fire's source. Investigators are trying to determine who owns the vehicle.

Meanwhile, the lightning-caused Halstead Fire burning in the Frank Church River of No Return Wilderness in central Idaho on Sunday grew slightly to 36 square miles.

Fire spokesman Bruce Palmer said crews are working to build a fire line that parallels State Highway 21 to make sure the travel corridor between Boise and Stanley remains open. He said the fire is within a few miles of the highway at one point but that's also where fire lines are most secure.

"That's a very big priority," he said, noting the local economy relies on tourist dollars from visitors to the scenic and mountainous area favored by outdoor enthusiasts.

About 375 firefighters are working on the fire that is expected to burn into the fall.

"It's going to take some kind season-ending weather event to stop the fire," Palmer said.

Elsewhere, the Grizzly Fire about seven miles north of Cambridge has burned 850 acres but is now 80 percent contained and full containment was expected late Sunday.