The authorities in Spokane County, Wash., arrested a man on Monday who they suspected of starting one of three fires that forced more than 65,000 people to evacuate over the weekend.

Aaron Farinacci, 37, was charged with one count of first-degree arson, according to Sheriff John Nowels of Spokane County. Sheriff Nowels said law enforcement had initially spoken with Mr. Farinacci on Saturday afternoon after police received a call from a resident of the area where the Old Trails fire began.

Fire investigators reported that the Old Trails fire appeared to be intentionally set by using a match or a butane lighter to ignite grass, the sheriff said. That blaze was one of the three — along with the Autumn Lane and Fairview fires — that ravaged parts of Spokane on Saturday and Sunday.

Mr. Farinacci was carrying waterproof matches and a butane lighter when he was arrested, Sheriff Nowels said at a Monday evening news conference.

The resident told police that he had seen a man bending down in some grass near West Euclid Road and North Old Trails Road in the suburbs north of Spokane, near the ignition site of the blaze, Sheriff Nowels said.

“At that point, the male stood up and looked very nervous, according to our citizen witness,” the sheriff said.

The witness returned home, he added, “and soon after saw smoke coming from the area.”

Deputies stopped Mr. Farinacci shortly after the call, about a mile and a half from where the fire began. They questioned him, confirmed his identity with the resident who had called the police, and released him because the fire was worsening and they were needed elsewhere.

Mr. Nowels said the deputies who initially stopped Mr. Farinacci knew little about the nascent fire investigation. Investigators had used footage from the deputies’ body cameras to identify Mr. Farinacci on Monday, the sheriff said.

More than 700 buildings have been destroyed by the fires, and rescue crews expect that number to rise. No fatalities have been reported, though the sheriff said earlier Monday that the department was still trying to contact 14 people who were involved in calls seeking evacuation assistance as the fires spread.

Mr. Farinacci is being held on bail set at $1 million. It was not immediately clear late Monday if he had any legal representation.

According to court records from Maricopa County, Ariz., Mr. Farinacci pleaded guilty to manslaughter and aggravated assault in 2012 after initially being charged with first-degree murder in the death of his father.

Preston McCollam, Spokane County’s prosecuting attorney, said the investigation was still active and that he might seek other charges. The authorities are seeking help from witnesses in the Spokane region.

“This arrest was the result of the diligent work of vigilant citizens who were noticing things that were different, who actually reported it,” he said.