During the 2016 presidential campaign the FBI obtained a secret warrant to monitor the communications of Carter Page, who was then serving as an adviser to Donald Trump, over concerns that Page was acting as an agent of Russia, according to a report from The Washington Post.
The article, which cites anonymous sources, says a Foreign Intelligence Surveillance Court judge granted the warrant to the FBI last summer.
To get the warrant, the FBI "definitely had to be able to show probable cause that [Page] was effectively acting as an agent of Russia," Post reporter Adam Entous tells NPR's Morning Edition.
"They knew from previous cases back in 2013 that Mr. Page had engaged with ... what he thought was a Russian diplomat [who] turned out to be an intelligence officer for Russia," Entous says. "That was one piece of the puzzle ... but that's just one piece. We don't have a full understanding of the intelligence that went into the request that ultimately the court approved."
"This is the clearest evidence so far that the FBI had reason to believe during the 2016 presidential campaign that a Trump campaign adviser was in touch with Russian agents," Entous and his colleagues wrote in their Post article.
Entous told NPR's Rachel Martin that "it's important to keep in mind we're talking about probable cause."
"He hasn't been found guilty," Entous said. "He may never be charged with anything."
He noted that the FBI was granted a FISA warrant to surveil former American diplomat Robin Raphel over probable cause that she might have been acting as a foreign agent, but that the allegation "turned out to be completely wrong."
"The fact that they were able to get a warrant is obviously a piece of the puzzle," Entous says. "It shows what the FBI was interested in. It's not the same thing as a conviction."
The extent of Page's influence within the Trump campaign is unclear. In March 2016, candidate Trump included Page's name on a "very short list" of his advisers, Entous says. More recently, President Trump and his associates have been downplaying Page's role in the campaign.
FBI Director James Comey confirmed last month that the agency has been investigating possible ties between the Trump campaign and Russia since last July, as part of a larger investigation into Russian attempts to meddle with the U.S. election, but said he could not provide any details about the investigation.
"The FISA court and its orders are highly secretive," The Associated Press notes. "Judges grant permission for surveillance if they agree there's probable cause that the target is an agent of a foreign power. Though the standard is a high bar to meet, applications are hardly ever denied."
"Despite being overseen by judges, [the applications for FISA warrants] are not examined in the way that a normal application for a search warrant is," NPR's Nina Totenberg reported in 2013.
The Washington Post notes that most counterintelligence probes do not result in criminal charges.
Page, who had previously announced he was willing to testify as part of the House Intelligence Committee's investigation, told the Post he has "nothing to hide."
"This confirms all of my suspicions about unjustified, politically motivated government surveillance," he said.
As a reminder, the House, Senate and FBI are all conducting investigations into Russia's actions during the 2016 presidential election. That includes the possibility that U.S. citizens — and particularly Trump advisers and associates — might have cooperated in Russian attempts to influence the election in Trump's favor.
Some investigators, particularly House Republicans, are also keenly interested in leaks of classified information about ties between Trump's team and Moscow. They're pursuing information about who released such information and why.
And the White House has repeatedly attempted to focus attention on Trump's widely rebutted allegations of illegal surveillance of Trump Tower, or claims of politically motivated "unmasking" of the names of Trump allies in intelligence documents.
The intelligence community has already concluded that Russia did meddle in the U.S. election. Open questions include how much it did, who knew about it and who may have cooperated.
The AP has more on why Page fell under suspicion:
"Page's relationship with Russia began to draw scrutiny during the campaign after he visited Moscow in July 2016 for a speech at the New Economic School. While Page said he was traveling in a personal capacity, the school cited his role in the Trump campaign in advertising the speech.
"Page was sharply critical of the U.S. in his remarks, saying Washington has a 'hypocritical focus on ideas such as democratization, inequality, corruption and regime change.'
"Days later, Page talked with Russia's ambassador to the U.S. at an event on the sidelines of the Republican National Convention. Attorney General Jeff Sessions spoke with the Russian envoy at the same event, a conversation he failed to reveal when asked about contacts with Russians during his Senate confirmation hearings. ...
"Page, a former Merrill Lynch investment banker who worked out of its Moscow office for three years, now runs Global Energy Capital, a firm focused on energy sectors in emerging markets. According to the company's website, he has advised on transactions for Gazprom and RAO UES, a pair of Russian entities."