The FBI can get a secret FISA warrant to investigate you as a foreign intelligence threat and uncover information that has nothing to do with foreign intelligence, but rather a domestic crime, and go after you for that. From Eric Tucker at apnews.com:
WASHINGTON (AP) — The case against Nassif Sami Daher and Kamel Mohammad Rammal, two Michigan men accused of food stamp fraud, hardly seemed exceptional. But the tool that agents used to investigate them was extraordinary: a secretive surveillance process intended to identify potential spies and terrorists.
It meant that the men, unlike most criminal defendants, were never shown the evidence authorities used to begin investigating them or the information that the Justice Department presented to obtain the original warrant.
The case is among recent Justice Department prosecutions that relied on the same surveillance powers, known by the acronym FISA, that law enforcement officials acknowledge were misused in the Russia investigation. Those errors have prompted a reckoning inside the FBI and debate in Congress about new privacy safeguards. The attention given to FISA has also cast a spotlight on cases such as the Michigan one, where surveillance tools used to investigate foreign intelligence threats end up leading to prosecutions for commonplace, domestic crimes.