Fire Marshal Steve Riggs said the wooden structure was too dangerous to allow firefighters inside so they poured on water from the outside.
The Skagit Valley Herald reports police arrested a person of interest on the roof as the fire broke out. He was questioned and released Sunday night. Police spokeswoman Shannon Haigh says he's a transient.
The Alf Christianson Seed building was vacated two years ago and has no electricity.
The 10-acre site has been considered as the location for a new Skagit County Jail. It sits along railroad tracks, and freight and passenger trains were delayed when fire hoses crossed the tracks.