“It is not common to use it in criminal proceedings, and in fact it’s somewhat controversial,” Aftergood said.

That’s because the bar for an FBI agent to get surveillance approval through the secret FISA court is lower than what would be needed to get a search or wiretap warrant through a regular court.

“To get a FISA authorization, you need to provide the court with probable cause that someone is an agent of a foreign power or a terrorist. But you don’t have to tell the judge they are probably engaged in criminal activity,” Aftergood said. “It’s a lower standard, and the concern is we don’t want the government using the lower standard to get warrants it could not obtain using the regular requirements for criminal warrants.”

If investigators do obtain evidence of a crime during FISA-approved surveillance, they can use it in a prosecution, but they have to put the defense on notice so they have the opportunity to challenge it.

Aftergood said courts usually approved the use of such evidence at trial.

“To my knowledge, a court has never said that evidence gathered through FISA collection is not approved. In every case I’m aware of, they say it’s acceptable,” he said.