Washington ? Firefighters strengthened their grip Monday on a 35,000-acre wildfire that had whipped through canyons, wheat fields and forestland in southeastern Washington, destroying dozens of homes and cabins.

Fire crews had good lines around three sides of the fire Monday, but remained concerned about heavy timber in the Umatilla National Forest to the south.

“Within the forest, it was a true firestorm,” said Clay Barr, director of emergency management for Garfield County.

Firefighters were still working to assess damage in areas still smoldering. While more than 100 structures were believed to have burned, about 20 of those were probably full-time homes, with the rest being hunting cabins, recreational homes and smaller structures.

The cause of the blaze remained under investigation, though fire officials believed it was not caused by lightning, said Don Ferguson, spokesman for the Northwest Fire Coordination Center.

So far this year, wildfires have charred 5.1 million acres, compared with 5.5 million at the same time last year, the center said.