An investigation by the Washington state Department of Natural Resources found that tree branches rubbing on a power line started the 2015 blaze near Twisp that killed three U.S. Forest Service firefighters.
The agency's report says ignition likely happened as a light wind moved the branches and a sagging Okanogan County Electric Cooperative distribution line on Aug. 19, 2015. The investigation says burning foliage then dropped to the ground and spread flames. The Seattle Times obtained the DNR document through a public records request.
A utility official told investigators the power lines are maintained to standards, with trees removed every three years or whenever deemed necessary. In the area where the fire began, the lines were last cleared during the 2012-13 winter.
The report didn't comment on the question of potential negligence.
"The fire danger rating for that area was categorized as very high, which indicated fires would start easily from all causes and immediately after ignition would spread rapidly and increase quickly in intensity," the report said.