Federal prosecutors say Ethan Kollie bought the armor, a gun accessory and a 100-round magazine for his friend Connor Betts earlier this year.
Investigators say there is no reason to believe that Kollie, who bought and stored the equipment, knew his friend was planning a mass shooting.
Betts used that equipment on Aug. 4, when he opened fire in the historic Oregon District in Dayton, Ohio, killing his sister and eight others.
Todd Wickerham of the FBI tells reporters Monday that it does not appear Kollie knew how Betts would use that equipment.
Prosecutors are accusing Kollie of lying on federal firearms forms. Those charges are not related to the shooting.