The Monroe County Sheriff's Office says 69-year-old Juan Zigler handed clerks at the Marathon post office a garbled note Friday that threatened to blow up the Keys.
They say he then threw an object over the counter that clerks thought might be a firecracker.
When deputies arrived, they arrested Zigler. The object appeared to be a battery with a wire attached to it.
Zigler was charged with making a false bomb report and delivering a hoax bomb and released.
No phone number for Zigler could be found Saturday.