If they had a FISA warrant (the reports said they did) then it actually smells more like a criminal conspiracy...particularly since Manafort is now being threatened with an indictment.
A "FISA warrant," issued by a special court established by Congress as part of the 1978
Foreign Intelligence Surveillance Act, does not require the government to show probable cause of a crime, as it would to get authority to conduct another form of search or surveillance. The government only needs to persuade one of the court's judges that there is reason to believe the target is acting as an agent of a foreign government. In other words, it appears that Manafort, along with Carter Page and possibly other advisors to the Trump campaign, was believed by the FBI to be engaged in some form of espionage—presumably as part of the Russian government's effort to influence the outcome of the presidential election.
Traditional warrants, such as the one issued in July to search Manafort's home in Virginia, do indicate that investigators have reason to believe the target engaged in criminal activity, and the New York Times reported yesterday that "[t]he special counsel, Robert S. Mueller III, then followed the house search with a warning: His prosecutors told Mr. Manafort they planned to indict him, said two people close to the investigation."
