Your browser does not support the audio element.

UPDATE (July 20, 7:15 a.m. PT) — The Substation Fire burning east of The Dalles, Oregon, is now the nation's top priority fire. That means it's first in line for national fire resources as needed and available.

THANKS TO OUR SPONSOR: Become a Sponsor

"This adds more people and tools to the 217 firefighters who are currently out here, and that's representing 73 fire agencies across our state," said Stefan Myers with the Oregon State Fire Marshal's Office.

Firefighting crews assemble to contain the Substation Fire burning east of The Dalles, Ore. Courtesy of Substation Fire Incident Management Team

On Friday morning, firefighting crews reported that the blaze had grown to 70,000 acres — up 20,000 overnight. But they also reported progress in controlling the fire, saying it is now 15 percent contained.

The Substation Fire started Tuesday and also led to one civilian fatality.

The man who died from exposure to the Substation Fire was trying to save a neighboring property from burning, and Wasco County officials appeared for the first time Thursday to suggest that the fire was human-caused.

Related: Can 'Moneyball' Fix How The West Manages Wildfire?

In a news release, the Wasco County Sheriff's Office identified 64-year-old Wasco County resident John Ruby as the man found dead near a burned tractor Wednesday afternoon. Investigators said Ruby was creating a fire line around a neighboring property to prevent the fire's advance.

Friends of Ruby's have set up a donation account for his relatives at Columbia Bank, which has branches across Oregon, Washington and Idaho.

The Wasco County Sheriff’s Office also said that the Substation Fire is “incendiary in nature” and is being investigated by county, state and federal officials. In fire investigations, incendiary is a technical term, meaning “a fire that is deliberately ignited under circumstances in which the person knows that the fire should not be ignited.” The term is not the same as arson.

Wasco County Sheriff Lane Magill referred to it Thursday as a "criminal investigation."

"The sheriff's office understands that you have many questions surrounding the ongoing criminal investigation related to the Substation Fire," Magill said. "However, answering those questions at this time could potentially compromise our active investigation."

The sheriff’s office didn’t offer more details on the fire’s cause. When asked by OPB whether “incendiary” had been used in its technical sense, a representative deferred questions to officials who weren’t immediately available.

THANKS TO OUR SPONSOR: Become a Sponsor

The press release marked the second time Thursday that an official had acknowledged the fire might have been started by humans. In a news conference earlier in the day, Gov. Kate Brown told reporters that the state was helping investigate the matter.

“Clearly you’re hearing that there is a likelihood of arson, so our agencies are going to help in that investigation,” Brown said.

The sheriff's office has set up a tip line for its investigation into the fire's start: 1-800-452-7888.

A burn scar near Moro, Oregon, from the Substation Fire. Ericka Cruz Guevarra / OPB

To date, the fire has killed Ruby and has imperiled harvests in one of the state's largest wheat regions. It also destroyed the historic Charles Nelson House, a popular photography destination near Dufur, Oregon.

In this heart of wheat country, where the economy relies almost entirely on agriculture and tourism, the can-do spirit that led to Ruby's death is not uncommon.

"All the ranchers, they all have firefighters on the back of their trucks and they go out and help one another," said Carey Hughes who owns Huskey's 97 Market in Moro, the lone grocery store in town. "And if they didn't, we would have a lot of lost homes and stuff around here."

Related: End Of An Icon: Historic Nelson House Burned In Substation Fire

Hughes said Thursday morning that a local resident came into the store saying the fire had flared up at the Deschutes River.

"And out they go," she said, referring to her customers.

Wheat accounts for about 91 percent of what's planted each year in Sherman County, making it one of the largest wheat producing-counties in Oregon. The grains are a top commodity in the Columbia Basin.

"This is the heart of wheat country," said Jim Johnson, land use and planning coordinator with the Oregon Department of Agriculture at a press conference Thursday morning. "They were just getting into harvest season. Lots of wheat is at risk."

The Substation Fire ignited Tuesday, growing rapidly that night as strong winds kicked up. It has torched land and threatens the towns of Moro and Grass Valley, in addition to a communications tower and Bonneville Power Administration power lines.

Hundreds of homes are threatened. As of Thursday morning, Level 3 "go now" evacuation notices have expanded to Grass Valley, Biggs Junction and Highway 30 from Deschutes State Park to Biggs. The fire has prompted evacuation notices for at least 900 homes in Wasco and Sherman counties.

Lightning, a frequent cause of wildfire, doesn’t appear to have been a factor in the Substation Fire. According to the Northwest Interagency Coordination Center, there were five lightning strikes in the Northwest from Tuesday morning until Wednesday morning. All of them occurred in north central Washington.

Molly Solomon contributed to this report.