Prosecutors said Harpham acted alone. He was arrested March 9 at his rural home near Addy, Wash.
The plea deal charged Harpham with attempted use of a weapon of mass destruction, and the hate crime of placing the bomb in an effort to target minorities. Prosecutors dropped charges of using a firearm in relation to a crime of violence and unauthorized possession of an unregistered explosive device. If convicted, he could have faced up to life in prison.
The Southern Poverty Law Center, which tracks hate groups, has said that Harpham made more than 1,000 postings on the Vanguard News Network, a white supremacist website. The center also has said that Harpham belonged to a neo-Nazi group called the National Alliance.
He has remained in the Spokane County Jail without bail since his arrest. Under the deal, Harpham would remain on probation for the rest of his life once he leaves prison.
During his sentencing, a suspicious package was found near the federal courthouse but a bomb squad determined it was not an explosive.