Fire crews are contending with summer heat and rough terrain as they fight to encircle a 49,000-acre wildfire, the largest in the lower 48 states.

The fire in southeastern Washington has destroyed more than 100 homes and more than 100 outbuildings about 15 miles south of Pomeroy (search). It was second in size only to an 82,000-acre blaze in Alaska.

Fire officials estimated the blaze was 75 percent contained Monday but could not say when it might be brought under control.

"We have two miles of line left to build but those are a couple of rugged miles," said spokesman Julian Rhinehart.

Rain is expected Wednesday, but there is also a chance of lightning, he said.

In western Montana, crews fighting an 11,000-acre blaze near Alberton were helped by cool, moist weather. That fire, which began as two blazes along Interstate 90, is nearly contained and wind forecast this week isn't expected to create any major problems, incident commander Bob Sandman said.

Full containment was predicted by Thursday, but it is likely to continue smoldering inside the fire lines for some time.

"You'll be eating Christmas dinner thinking 'Is this fire out yet?"' Sandman told residents at a town meeting in Alberton.

All the evidence so far shows arson is unlikely, said Laura Mark, assistant special agent for the U.S. Forest Service (search). Mark said the fires probably were started by sparks from a malfunctioning vehicle. One man was being questioned but is not a suspect, authorities said.

Crews in Idaho also are making progress. A blaze that had threatened 80 homes near Grangeville since last week was estimated at 4,800 acres Monday and was 40 percent contained, said Shannon Downey, a spokeswoman for firefighters. No homes have burned and the blaze is expected to be completely contained by Saturday, she said.

The National Interagency Fire Center (search) said 28 large fires totaling 290,000 acres were active Monday in Alaska, Arizona, Arkansas, California, Idaho, Minnesota, Montana, Nevada, Oregon and Washington. So far this year, wildfires have charred 5.89 million acres nationwide, compared with 5.95 million acres at the same time last year.