The FBI and Justice Department demonstrated probable cause that Page is acting on behalf of a foreign state in order to be granted the warrant.
The FISA warrant was part of the FBI's investigation into possible ties between Russia and Trump campaign associates, law enforcement and U.S. officials told the Post.
House, FBI and Justice Department declined the Post's request for comment.
This report indicates that the FBI believed Page was in touch with Russian officials during the 2016 election.
House and Senate Intelligence committees both have ongoing investigations into whether Trump associates worked with the Kremlin in an attempt to help Trump win the election.
Page has been at the center of speculation about links between Trump aides and Russia because of his ties to the country and a trip to Moscow in July 2017.
The government has not formally charged Page with any wrongdoing, and it is unclear if the Justice Department plans to do so.
The Washington Post first reported last week that Page admitted that he met with a Russian spy in 2013, but the former Moscow-based investment banker denies any wrongdoing with regard to Russia and the Trump campaign.
Trump identified Page as a foreign policy adviser in the spring of 2016.
A FISA warrant is granted for a 90-day span, and this one was renewed more than once, officials told the Post.
FISA grants are given sparingly, quietly, and require the approval from top Justice Department and FBI officials.
The FISA application for Page laid out in a lengthy testimony claims about how he may be linked to the Russian government or an agent who is carrying out clandestine actions for the Kremlin, an official told the Post. A crime does not need to be committed in order to receive a warrant. But information obtained through a FISA warrant could be used in later prosecution.
The warrant application also reportedly claimed Page was in communication with other Russian agents whose identities have not yet been publicly disclosed, in addition to his contacts that have already been reported.
The FISA reveals that the FBI and Justice Department both worked to uncover Russian ties to the election as early as July, an official told the newspaper.
Page reportedly was the only U.S. citizen that the government directly surveilled in 2016 through a FISA warrant as part of the government's Russia probe.
Page raised concern among foreign policy experts for publicly and effusively praising Russian President Vladimir Putin and criticizing U.S. foreign policy towards Russia. In July, Page traveled to Moscow to deliver a speech on the topic.
The Post reports that the FBI and the Justice Department felt reluctant to use a FISA warrant on campaign figures during the election because it could lead to intercepted communications regarding political campaign strategy.
Obtaining a FISA warrant to surveil foreign diplomats, however, is much more common. The Post reported the FBI used a FISA warrant to listen to Russian Ambassador Sergey Kislyak's conversations with Gen. Michael Flynn, Trump's former national security adviser who was ousted after details of the talks became public.
Who is Carter Page?
Former foreign policy adviser for the Trump campaign, energy executive and oil industry consultant. Page had worked in Moscow for years as a vice president with Merrill Lynch and made recent trips to Russia.
Russian business connections:
2004-2007 | Page runs the Moscow office of Merrill Lynch. He said he advised the state-run energy conglomerate Gazprom and other energy companies. However, Page's then-supervisor said Page's role was to execute deals rather than to negotiate or advise on them. Read more
March 2016 | Page is hired by the Trump team. In an interview with Bloomberg News, he says he owns shares of Gazprom and that his stock portfolio had suffered since the United States and Europe imposed economic sanctions on Russia. Read more
July 7, 2016 | Page gives a speech critical of U.S. policy toward Russia on a Moscow trip that had been approved by Trump's campaign manager on the condition that Page not formally represent the campaign. While there, Page allegedly met with Igor Sechin, a Putin confidant and chief executive of the energy company Rosneft, according to a dossier cited by Rep. Adam Schiff, the ranking Democrat on the House intelligence committee. Some of the information in the dossier has been verified by U.S. intelligence agencies, while other parts have proved false. Read more
July 18, 2016 | Russian Ambassador Sergey Kislyak speaks with Page and campaign adviser J.D. Gordon following a panel at the Republican National Convention, where Sen. Jeff Sessions (R-Ala.) had also met Kislyak. Read more
Sept. 26, 2016 | Page leaves the campaign after reports showed he met with Igor Sechin, the head of state-owned Russian oil giant Rosneft, and other high-ranking Russian officials during the campaign. Page denies meeting “any of those guys” and volunteers to testify in front of Congress to clear his name. Read more
April 3, 2017 | BuzzFeed reports Page met with a Russian agent named Victor Podobnyy in 2013 in New York at an energy conference, according to court filings. Page later tells The Washington Post that the reason his name came up in the court filings was because he was helping the Justice Department build its case against Evgeny Buryakov, an undercover Russian agent who was posing as a bank executive in New York at Vnesheconombank. Buryakov was later convicted of foreign espionage.
Carter Page, a former foreign policy adviser of President Donald Trump, speaks at a news conference at the RIA Novosti news agency in Moscow in December. Page said he was in Moscow to meet with businessmen and politicians.
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.
In order 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."
Listen: Adam Entous on FBI's FISA Court Warrant To Monitor Page
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"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 they did, who knew about it and who may have cooperated.
The AP has more on why Carter 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 energy companies.