The agents sold 19-year-old Mohamed Osman Mohamud fake explosives and set him up on a sting operation.
Mohamud was arrested just after 5:30 p.m. Friday when he tried dialing a cellular phone he thought would set off the explosives.
Instead, the call brought federal agents and police swooping down to arrest him.
The FBI said Mohamud was planning the attack for months, and at one point, he mailed bomb components to FBI operatives whom he believed were assembling the device.
Mohamud, a naturalized U.S. citizen living in Corvallis, was charged with attempted use of a weapon of mass destruction, which carries a maximum sentence of life in prison. A court appearance was set for Monday.
The Associated Press contributed to this report.