John T. Booker, Jr., 20, of Topeka, had arrived at the Kansas base with two undercover FBI agents to detonate what he did not realize was an inert bomb, prosecutors said.
Booker was charged with three criminal counts including attempting to use a weapon of mass destruction and attempting to provide material support to Islamic State fighters, who have captured parts of Iraq and Syria over the past year and has sympathizers in several countries.
A second Topeka man, Alexander Blair, 28, was arrested and charged on Friday with failing to report a felony. Blair shared some of Booker's views, knew of his intent to detonate a bomb at Fort Riley and loaned Booker money to rent a storage unit Booker used to store bomb components, prosecutors said.