The FBI obtained a secret court order mid last year to monitor the communications of an adviser to presidential candidate Donald Trump, part of an investigation into possible links between Russia and the campaign, law enforcement and other US 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 campaign adviser was in touch with Russian agents. Such contacts are now at the centre of an investigation into whether the campaign coordinated with the Russian Government to swing the election in Trump's favour.

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 US elections began in July, officials have said. Most such investigations don't result in criminal charges.

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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 today. "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.

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

FBI Director James 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 America's most sensitive national security cases, and their warrants are some of the most closely guarded secrets in the world of US 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 another Russian agent. In addition, the application said Page had other contacts with Russian operatives that have not been publicly disclosed, officials said.

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 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 northern spring and summer of 2016, coming from New York to Washington to meet with the group. Although Trump did not attend, Senator Jeff Sessions, a top Trump confidant who became his attorney general, attended one meeting of the group with Page, 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 US sanctions over Moscow's military intervention in Ukraine.

In July, Page travelled 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 Congressman Adam Schiff, the ranking Democrat on the House Intelligence Committee. Officials said some of the information in the dossier has been verified by US intelligence agencies, and some of it hasn't, while other parts are unlikely to ever be proved or disproved.