US authorities have arrested a 37-year-old man on suspicion of starting the Old Trails Fire, one of three uncontained wildfires that destroyed at least 700 structures and forced about 67,000 people to evacuate around Spokane.

The Spokane County Sheriff’s Office said Aaron F. Farinacci was arrested on Monday, 3 August, after detectives and Washington state fire investigators developed probable cause to suspect him of first-degree arson.

Farinacci was arrested without incident while officers executed a search warrant at his home, according to the sheriff’s office. He was booked into the Spokane County Jail and was being held on a $1 million bond, the Associated Press reported.

It was not immediately clear whether he had a lawyer. The allegation against him has not been tested in court.

“There’s nothing to save,” Miriam Sim, 76, told the AP after returning with her husband to the ruins of the Spokane home where they had lived since 2005.

The Old Trails Fire was reported at about noon on Saturday, 1 August, near North Old Trails Road and West Euclid Road, north of Airway Heights. The sheriff’s office said information from residents, local police and deputies involved in the initial evacuations helped investigators identify Farinacci.

The arrest concerns only the Old Trails Fire. The causes of the Autumn Lane and Fairview fires remain under investigation.

The three fires had burned about 8,000 acres — approximately 32 square kilometres — by Monday and were all listed as 0% contained. More than 1,000 firefighters had been deployed, with additional crews expected to arrive.

At least 700 buildings, most of them in residential areas, had been destroyed, officials told Reuters. Infrared surveys indicated that as many as 400 more structures could have been damaged or destroyed, although that estimate had not been confirmed.

No deaths or major injuries had been reported by Monday. Spokane County initially received nearly 300 reports of missing people but had cleared all except 14, according to the sheriff’s office.

Officials said those outstanding reports were believed to involve people who had not contacted relatives since evacuating, rather than confirmed missing people.

About 400 displaced residents were staying at the Spokane Convention Center, which had been converted into an emergency shelter. Other shelters were operating for residents and animals affected by the fires.

Cooler temperatures and calmer winds helped crews on Monday, but fire managers warned that hotter and drier conditions would return during the middle of the week.

Governor Bob Ferguson declared a statewide wildfire emergency and imposed a burn ban. Washington’s congressional delegation also asked President Donald Trump to approve an expedited federal emergency declaration.

By early Tuesday, the National Weather Service in Spokane said wildfire smoke was expected to persist across eastern Washington and northern Idaho throughout the week. Damage assessments and investigations into the Autumn Lane and Fairview fires were continuing.