None of the museum's main exhibits were damaged, said Brian Sajko, curator of the museum that honors the college's famous graduate. He said most of the damage was in a locked storage closet where some of the former president's memorabilia was stored.
Eureka Fire Chief Craig Neal said state investigators ruled Monday's fire was set in the storeroom.
A sprinkler system helped quell flames but the fire left smoke and water damage, destroying some trophies, papers and a large portrait of the actor-turned-president, Sajko said.
Of more than 3,000 items in the college's collection, only about 1,000 are on display, including Reagan's 1932 Eureka diploma, an essay he wrote as a student and photos from his acting and political careers.