The suspect, Spokane resident Aaron Farinacci, 37, previously imprisoned for killing his father, made his initial appearance in county court after being jailed on a $US1 million bond on a count of first-degree arson, according to court records.
Farinacci is accused of starting the Old Trails Fire, one of three that erupted on Saturday to the north and west of Spokane, by using matches or a lighter to ignite a roadside patch of grass, Spokane County Sheriff John Nowels told reporters.
The origins of the two other fires remained under investigation, he said.
The three blazes together are ranked as the top firefighting priority in the US as a whole. Dozens of major fires have scorched Washington and Oregon for weeks, fouling the air with smoke and soot.
Driven by gusting winds and tinder-dry conditions, the Spokane-area fires have blackened more than 4000ha on the northern fringe of the city, home to about 230,000 residents.
Online drone footage showed the Old Trails Fire swept out of a state park into wooded, suburban neighbourhoods, turning homes to ash, melting cars and leaving only blackened trees standing.
Authorities pleaded with people not to fly drones after dozens of such illegal flights grounded all firefighting aircraft.
"We're still in a firefight here," said Tom Stokesberry, spokesman for firefighting teams in the area. "This is just a dynamic situation with everything we're facing, three separate fires in the urban interface."
Cooler temperatures and lighter winds were expected to help firefighters on Tuesday, Stokesberry said, adding the Old Trails and Fairview fires remained stable in size while the Autumn Lane fire continued to grow.
Temperatures were set to rise to 32C later this week, making it harder to control the blazes which are not contained at all, state officials said.
No fatalities had been reported from the Spokane fires, Stokesberry said.
Nowels said the arson suspect was first detained on Saturday after a witness reported seeing a nervous-looking man bending down near some grass at the spot where arson investigators later determined the Old Trails Fire began.
Farinacci was questioned and released that day, but taken into custody on Monday afternoon, once investigators had obtained further evidence, Nowels said.
In 2010, Farinacci shot dead his father in Mesa, Arizona, after an argument over washing dishes, police said. He subsequently shot himself but survived.
Farinacci pleaded guilty to manslaughter and was sentenced to 12 years imprisonment. He was released from custody in 2020, according to court records. It was not immediately known whether Farinacci had obtained legal representation on the fire charge.
Stokesberry said an additional 1100 firefighters were en route to Spokane to double the force seeking to fully suppress the fires.
They are among some 27,400 personnel fighting wildfires nationwide, mainly in Oregon, Washington and Idaho, according to the National Interagency Fire Centre in Boise.
The centre has tallied 45,050 wildfires nationwide year to date, the highest such number in at least a decade.
