The forest fire that started Saturday southwest of Custer, signaling the start of the 2008 fire season, is believed to have been caused by a person and was under investigation Sunday.

Frank Carroll, public affairs officer for the Black Hills National Forest, said officials believe it was caused by a person because there had been no lightning in the area for several days, but they don't know whether it was set deliberately or accidentally.

"We have really well-trained fire investigators who are looking into it," he said.

As many as 150 firefighters battled the blaze Sunday.

Firefighters worked overnight Saturday to bulldoze fire lines around it, but wind gusts of up to 28 mph Sunday afternoon meant control of the fire was uncertain, Carroll said.

The Freeland Fire is burning in grass and ponderosa pine in Pass Creek, a heavily forested area broken by meadows and brush fields of mountain mahogany, Carroll said.

It burned mostly grass Saturday but jumped to some trees over Saturday night, then Sunday was contained to grass and smoldering stumps and fallen logs.

The heavy snowfall from the blizzard two and a half weeks ago is gone, and the grass appears green but contains fuel in the form of dead grass from last year and pine needles, Carroll said.

Firefighters worked to keep the blaze contained behind the fire line, so it would exhaust the fuel and extinguish itself.

They worked alone and in teams of two along the fire line to extinguish embers. In the teams, one firefighter used a 40-pound, five-gallon backpack pump to spray embers while the other turned over the grass and dirt.

Incident commander Jay Miller is leading the crews and manning the command post, a trailer with computers and GPS systems to monitor the terrain and blaze. Miller said the crews were trying to extinguish the fire by today. He said air tankers would be brought in if conditions worsened.

Also on scene was Gary Lipp, a veteran firefighter and fire management officer for the Hell Canyon District. He's been working with younger firefighters, Carroll said, teaching them skills such as how to adapt to changing weather conditions and what to do when burning material rolls across fire lines.

Local crews from Hot Springs and Argyle are working with state firefighters from the Black Hat and Bear Mountain handcrews and the Black Hills National Forest's Tatanka Hotshots, who were called back from work in Colorado.

No structures or residents were threatened.

The public is asked to stay away from the fire area. Residents are cautioned that there is heavy fire-related traffic on Forest Road 273.

Carroll said with the buildup of dense fuel in the Hills, people should be careful about not accidentally starting a fire: Don't toss cigarette butts and beware of parking on dry grass, where a catalytic converter can start a fire.