The building burned early Monday. Dennis Wagner, a detective with the Oregon State Police, said a dog trained to sniff out flammable liquids was sent in, but said other evidence led to the conclusion of arson.
He declined to elaborate but said there are no suspects.
He said the Hermiston Fire Department was able to save enough of the building to preserve helpful evidence.
He said a U.S. Bureau of Alcohol, Tobacco and Firearms agent and a Hermiston Police officer canvassed the area seeking witnesses and that police are looking at security tapes from the area for leads.
Insurers are offering a $5,000 reward for the arrest and conviction of those responsible.
Damage to the court facility was estimated at about $200,000. The entire inventory of an adjacent clothing store was ruined by smoke and water.
