The FBI obtained a secret court order in October 2016 to monitor the communications of a former adviser to presidential candidate Donald Trump, part of an investigation into possible links between Russia and the campaign, law enforcement and other U.S. officials said.

The FBI and the Justice Department obtained the warrant targeting Carter Page’s communications after convincing a Foreign Intelligence Surveillance Court judge that there was probable cause to believe Page was acting as an agent of a foreign power, in this case Russia, according to the officials.

This is the clearest evidence so far that the FBI had reason to believe during the 2016 presidential campaign that a Trump associate was in touch with Russian agents. Such contacts are now at the center of an investigation into whether the campaign coordinated with the Russian government to swing the election in Trump’s favor.

Page has not been accused of any crimes, and it is unclear whether the Justice Department might later seek charges against him or others in connection with Russia’s meddling in the 2016 presidential election. The counterintelligence investigation into Russian efforts to influence U.S. elections began in July, officials have said. The FBI also began investigating Page in July as part of the probe, officials said. Most such investigations don’t result in criminal charges.

The officials spoke about the court order on the condition of anonymity because they were not authorized to discuss details of a counterintelligence probe.

During an interview with the Washington Post editorial page staff in March 2016, Trump identified Page, who had previously been an investment banker in Moscow, as a foreign policy adviser to his campaign. Campaign spokeswoman Hope Hicks later described Page’s role as “informal.”

Page has repeatedly denied any wrongdoing in his dealings with the Trump campaign or Russia.

“This confirms all of my suspicions about unjustified, politically motivated government surveillance,” Page said in an interview Tuesday. “I have nothing to hide.” He compared surveillance of him to the eavesdropping that the FBI and Justice Department conducted against civil rights leader Martin Luther King Jr.

[Despite early denials, growing list of Trump camp contacts with Russians haunts White House]

The White House, FBI and Justice Department declined to comment.

FBI Director James B. Comey disclosed in public testimony to the House Intelligence Committee last month that the bureau is investigating efforts by the Russian government to interfere in the 2016 presidential election.

Comey said this includes investigating the “nature of any links between individuals associated with the Trump campaign and the Russian government and whether there was any coordination between the campaign and Russia’s efforts.”

Comey declined to comment during the hearing about any individuals, including Page, who worked in Moscow for Merrill Lynch a decade ago and who has said he invested in Russian energy giant Gazprom. In a letter to Comey in September, Page had said he had sold his Gazprom investment.

During the hearing last month, Democratic lawmakers repeatedly singled out Page’s contacts in Russia as a cause for concern.

The judges who rule on Foreign Intelligence Surveillance Act (FISA) requests oversee the nation’s most sensitive national security cases, and their warrants are some of the most closely guarded secrets in the world of U.S. law enforcement and intelligence gathering. Any FISA application has to be approved at the highest levels of the Justice Department and the FBI.

Applications for FISA warrants, Comey said, are often thicker than his wrists, and that thickness represents all the work Justice Department attorneys and FBI agents have to do to convince a judge that such surveillance is appropriate in an investigation.

The government’s application for the surveillance order targeting Page included a lengthy declaration that laid out investigators’ basis for believing that Page was an agent of the Russian government and knowingly engaged in clandestine intelligence activities on behalf of Moscow, officials said.

Among other things, the application cited contacts that he had with a Russian intelligence operative in New York City in 2013, officials said. Those contacts had earlier surfaced in a federal espionage case brought by the Justice Department against the intelligence operative and two other Russian agents. In addition, the application said Page had other contacts with Russian operatives that have not been publicly disclosed, officials said.

[Former Trump adviser admits to 2013 communication with Russian spy]

An application for electronic surveillance under the Foreign Intelligence Surveillance Act need not show evidence of a crime. But the information obtained through the intercepts can be used to open a criminal investigation and may be used in a prosecution.

The application also showed that the FBI and the Justice Department’s national security division have been seeking since July to determine how broad a network of accomplices Russia enlisted in attempting to influence the 2016 presidential election, the officials said.

Since the 90-day warrant was first issued, it has been renewed more than once by the FISA court, the officials said.

In February, Page told “PBS NewsHour” that he was a “junior member of the [Trump] campaign’s foreign policy advisory group.”

A former Trump campaign adviser said Page submitted policy memos to the campaign and several times asked to be given a meeting with Trump, though his request was never granted. “He was one of the more active ones, in terms of being in touch,” the adviser said.

The campaign adviser said Page participated in three dinners held for the campaign’s volunteer foreign policy advisers in the spring and summer of 2016, coming from New York to Washington to meet with the group. Although Trump did not attend, Sen. Jeff Sessions (R-Ala.), a top Trump confidant who became his attorney general, attended one meeting of the group with Page in late summer, the campaign adviser said.

Page’s role as an adviser to the Trump campaign drew alarm last year from more-established foreign policy experts in part because of Page’s effusive praise for Russian President Vladimir Putin and his criticism of U.S. sanctions over Moscow’s military intervention in Ukraine.

In July, Page traveled to Moscow, where he delivered a speech harshly critical of the United States’ policy toward Russia.

While there, Page allegedly met with Igor Sechin, a Putin confidant and chief executive of the energy company Rosneft, according to a dossier compiled by a former British intelligence officer and cited at a congressional hearing by Rep. Adam B. Schiff (Calif.), the ranking Democrat on the House Intelligence Committee. Officials said some of the information in the dossier has been verified by U.S. intelligence agencies, and some of it hasn’t, while other parts are unlikely to ever be proved or disproved.

On Tuesday, Page dismissed what he called “the dodgy dossier” of false allegations.

Page has denied such a meeting occurred, saying he has never met Sechin in his life and that he wants to testify before Congress to clear his name. A spokesman for Rosneft told Politico in September that the notion that Page met with Sechin was “absurd.” Page said in September that he briefly met Russian Deputy Prime Minister Arkady Dvorkovich during that trip.

Comey has declined to discuss the details of the Russia probe, but in an appearance last month, he cited the process for getting FISA warrants as proof that the government’s surveillance powers are very carefully used, with significant oversight.

“It is a pain in the neck to get permission to conduct electronic surveillance in the United States. And that’s good,’’ he told an audience at the University of Texas in Austin.

Officials have said the FBI and the Justice Department were particularly reluctant to seek FISA warrants of campaign figures during the 2016 presidential race because of concerns that agents would inadvertently eavesdrop on political talk. To obtain a FISA warrant, prosecutors must show that a significant purpose of the warrant is to obtain foreign intelligence information.

[How hard is it to get an intelligence wiretap? Pretty hard.]

Page is the only American to have had his communications directly targeted with a FISA warrant in 2016 as part of the Russia probe, officials said.

The FBI routinely obtains FISA warrants to monitor the communications of foreign diplomats in the United States, including the Russian ambassador, Sergey Kislyak. The conversations between Kislyak and Michael Flynn, who became Trump’s first national security adviser, were recorded in December. In February, The Washington Post reported that Flynn misled Vice President-elect Mike Pence and others about his discussions with Kislyak, prompting Trump’s decision to fire him.

In March, Trump made unsubstantiated claims about U.S. surveillance of Trump Tower in New York. Later that month, Rep. Devin Nunes (R-Calif.), chairman of the House Intelligence Committee and a Trump transition official, charged that details about people “associated with the incoming administration, details with little apparent foreign intelligence value” were “widely disseminated” in intelligence community reporting. He said none of the surveillance was related to Russia. The FISA order on Page is unrelated to either charge.

Last month, the former director of national intelligence, James R. Clapper Jr., told NBC’s “Meet the Press” that U.S. law enforcement agencies did not have any FISA orders to monitor the communications of Trump, either as a candidate or as a president-elect, or his campaign. But Clapper did not address whether there were any FISA warrants targeting Trump associates.

Three years before Page became an adviser to the Trump campaign, he came to the attention of FBI counterintelligence agents, who learned that Russian spy suspects had sought to use Page as a source for information.

In that case, one of the Russian suspects, Victor Podobnyy — who was posing as a diplomat and was later charged by federal prosecutors with acting as an unregistered agent of a foreign government — was captured on tape in 2013 discussing an effort to get information and documents from Page. That discussion was detailed in a federal complaint filed against Podobnyy and two others. The court documents in that spy case only identify Page as “Male 1.’’ Officials familiar with the case said that “Male 1’’ is Page.

In one secretly recorded conversation, detailed in the complaint, Podobnyy said Page “wrote that he is sorry, he went to Moscow and forgot to check his inbox, but he wants to meet when he gets back. I think he is an idiot and forgot who I am. Plus he writes to me in Russian [to] practice the language. He flies to Moscow more often than I do. He got hooked on Gazprom thinking that if they have a project, he could rise up. Maybe he can. I don’t know, but it’s obvious that he wants to earn lots of money.’’

The same court document says that in June 2013, Page told FBI agents that he met Podobnyy at an energy symposium in New York, where they exchanged contact information. In subsequent meetings, Page shared with the Russian his outlook on the state of the energy industry, as well as documents about the energy business, according to the court papers.

In the secret tape, Podobnyy said he liked the man’s “enthusiasm” but planned to use him to get information and give him little in return. “You promise a favor for a favor. You get the documents from him and tell him to go f--- himself,’’ Podobnyy said on the tape, according to court papers.

Page has said the information he provided to the Russians in 2013 was innocuous, describing it as “basic immaterial information and publicly available research documents.” He said he had assisted the prosecutors in their case against Evgeny Buryakov, who was convicted of espionage.

Rosalind S. Helderman contributed to this report.

Correction: This story has been updated to note that government investigators obtained a FISA warrant on Carter Page in October 2016, not the summer of 2016 as originally reported, and that in October 2016 Page was no longer an adviser to the Trump campaign.

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