What is a FISA warrant? What to know about the surveillance of Carter Page

AP

In the summer of 2016, the FBI was covertly granted a court order that allowed the agency to conduct surveillance on Carter Page, one of then-Republican presidential candidate Donald Trump's advisers.

According to United States law enforcement officials who spoke to the Washington Post on condition of anonymity, the warrant was issued after the Justice Department was able to adequately convince a Foreign Intelligence Surveillance Court judge that there was "probable cause to believe Page was acting as an agent of a foreign power."

So what does that mean, exactly?

As the Post notes, the fact of the warrant's issuance is the strongest evidence thus far that the FBI had reason to believe the Trump campaign was communicating inappropriately with a "foreign power" — in this case, Russia — during the 2016 campaign.

The FISA court and the orders it hands down are top secret business. As Time reports, warrants permitting surveillance are granted as long as there's probable cause that the target is an agent of a foreign power — and they are hardly ever denied.

According to Fox News, contact Page had with a Russian operative who tried to recruit him as an intelligence source in 2013 was enough to get the government's application for the FISA warrant approved.

But while officials launched the investigation into the Trump campaign's communications in July 2016, Page has yet to receive any formal charges of improper contact, and has himself repeatedly denied the allegations.

"I have done nothing wrong," he told Fox News in an interview. "I have no concerns."