[florean]

They did not. They told a government contractor that the document had been printed and mailed from Augusta, GA, who then reported it to the government. That's what got her, microdots or not.

You can read the search warrant at https://www.buzzfeed.com/stevenperlberg/a-federal-government..., which is more complete than the arrest warrant. Just read paragraphs 12-19, which cover the relevant probable cause. If the FBI don't know how it leaked (printed) and from where (Winner's home town), the case becomes extremely difficult. Instead, they had so much that she just confessed when they showed up.