Authorities say Abdullatif Ali Aldosary researched bomb-making materials and gathered chemicals before detonating the explosive outside the office in Casa Grande on November 30. No one was injured in the blast about 80 kilometres south of Phoenix.
The 47-year-old suspect initially faced five federal counts, including weapons and explosives charges, to which he has pleaded not guilty.
Federal authorities recently dismissed two of the charges related to the bombing, but continue to pursue the weapons charges.