Fire investigators say a wildfire that destroyed 11 homes in Karlstad, Minn., last week appears to have been deliberately set.

The Department of Natural Resources says the fire is still being investigated, and a $6,000 reward is being offered for information that helps identify the person who set it.

Curt Cogan, DNR forestry program coordinator and manager of the investigation, said the suspicion is based on evidence found at the origin of what became known as the County 27 fire, rather than witnesses or calls from the public. Investigators have simply ruled out other causes, such as lightning, a passing train or working farm equipment, Kittson County Sheriff Kenny Hultgren said.

"When you have no known cause other than somebody lighting it, that's when you have arson," Hultgren said.

It was one of at least eight fires that blazed across the extremely dry, flat and grassy landscape of northwest Minnesota in high winds last week. Cogan said it's possible some of the other fires might also have been set; they're being investigated, as well.

On average, 29 percent of fires on Minnesota land in rural areas not protected by local fire departments are purposely set, according to DNR figures. In 2011, it was 30 percent of 143 fires.

Those numbers include fires that might have been originally set to control brush or for some other private purpose, as well as those set out of maliciousness, said Carson Berglund, a spokesman at the Minnesota Interagency Fire Center.

The County 27 fire started at 2 p.m. Sept. 30, a Sunday, at the intersection of Kittson County Hwy. 27 and 250th Avenue NW, about 5 miles southwest of Karlstad. A day later, it grew to 500 acres and ultimately burned 4,440 acres, forcing a partial evacuation of Karlstad.

Hultgren said many arson fires in Kittson County begin on Sunday afternoons.

Why?

"I have no idea," he said.

"We're going to work this one vigorously," he added.

Two days after the fire started, firefighters were battling it on the south side of Karlstad. That forced the evacuation of the 69 residents of a nursing home and assisted living center, as well as other homes and businesses. Embers blew into town and in some cases set piles of leaves on fire, but firefighters were able to drive the fire around the west and east sides of town, Hultgren said.

Hundreds of firefighters are still working across the region, including 180 on the 4,400-acre North Minnie fire, north of Red Lake, which Wednesday was 35 percent contained. Gil Knight, spokesman for the interagency team working on that fire, said it's been difficult to get firefighters and equipment to the fire's north side, which is in an area with few roads.

Also, the North Minnie fire, as well as the Viking fire, near Thief River Falls, have dropped below ground into a layer of peat. Peat fires burn in low oxygen conditions without flame, can move laterally underground and are hard to put out. Officials have indicated they could burn for months.

In some places, firefighters are slogging through the remains of deep snow that fell across the region last week. Wednesday, they were also working in drizzle and temperatures in the low 40s, raising the prospect of hypothermia, Interagency Fire Center spokeswoman Lynn Mizner said.

Bill McAuliffe • 612-673-7646