One of Donald Trump's ex-foreign policy aides says allegations that he worked with the Russian government during the 2016 US election are "misleading".
The FBI believed Carter Page was "collaborating and conspiring with the Russian government" at that time.
His alleged relationships with Russian intelligence officials are highlighted in court applications which led to him being put under surveillance.
Mr Trump said it appeared that his campaign was illegally spied on.
But he provided no evidence to support the claim.
The newly released surveillance applications were granted and renewed by several different judges sitting in a court authorised by the Foreign Intelligence Surveillance Act (Fisa).
The court, which has the power to authorise wiretaps and surveillance of suspected foreign spies, is one of the most secretive institutions in the US.
What was released?
The FBI released the previously secret document cache on Saturday night following Freedom of Information requests by several US organisations.
It contains 412 pages of heavily redacted material which includes the surveillance applications, their later renewals, and warrants surrounding the investigation into Mr Page.
"The FBI believes that Page has been collaborating and conspiring with the Russian government," the October 2016 application to the court states.
According to the documents, "the FBI believes that the Russian government's efforts are being co-ordinated with Page and perhaps other individuals associated with" Mr Trump's presidential campaign.
It also said Mr Page "has established relationships with Russian government officials, including Russian intelligence officers".
Mr Page told CNN that describing him as an adviser to the Russian government was "way over the top".
"I've never been an agent of a foreign power by any stretch of the imagination," he said.
"You talk about misleading the courts, it's just so misleading. It's literally a complete joke."
Asked about his previous comments referring to himself as an informal adviser to the Kremlin, Mr Page responded that his contacts had been "informal, having some conversations with people... this is really nothing and just an attempt to distract from the real crimes shown in this misleading document".
Who is Carter Page?
Mr Page is an energy industry consultant with longstanding ties to Russia. He first contacted the Trump campaign in 2015 before meeting Mr Trump's campaign manager, Corey Lewandowski, in January 2016.
By March 2016, Mr Trump had identified Mr Page as one of a handful of campaign foreign policy advisers.
However, after allegations emerged in the Steele dossier that he had served as an intermediary between Russian officials and the president's campaign as they worked to advance Mr Trump's presidential efforts, Mr Page resigned from his role. He denied all the allegations against him.
Row over court 'politicisation'
Republicans have criticised the FBI for using material from the Steele dossier in applications to the Fisa court - describing its contents as entirely without foundation - while not telling the court that the Democratic National Committee and Hillary Clinton's presidential campaign had helped fund the research.
Democrats point out that the applications contain evidence against Mr Page that is not related to the Steele dossier, and that the FBI clearly told the court that the research sponsor wanted to discredit Mr Trump's presidential campaign.
The application said that based on the dossier author's previous reliable reporting history with the FBI, the agency believed the information to be credible.
What did Trump say?
The president took to Twitter to welcome the release of the documents and accuse the Department of Justice and the FBI of breaking the law to mislead the courts and spy on his campaign - to benefit his Democratic opponent in the election, Hillary Clinton.
Congratulations to @JudicialWatch and @TomFitton on being successful in getting the Carter Page FISA documents. As usual they are ridiculously heavily redacted but confirm with little doubt that the Department of “Justice” and FBI misled the courts. Witch Hunt Rigged, a Scam!— Donald J. Trump (@realDonaldTrump) July 22, 2018
Looking more & more like the Trump Campaign for President was illegally being spied upon (surveillance) for the political gain of Crooked Hillary Clinton and the DNC. Ask her how that worked out - she did better with Crazy Bernie. Republicans must get tough now. An illegal Scam!— Donald J. Trump (@realDonaldTrump) July 22, 2018
The leader of the Democratic Party in the US House of Representatives, Nancy Pelosi, came to a different conclusion.
In a statement, she said: "Despite President Trump's repeated claims, these documents provide clear evidence of 'Russia's co-ordination with Carter Page', a high-ranking Trump campaign official, 'to undermine and improperly and illegally influence the 2016 US presidential election'."
She added: "The GOP [Republican Party] must cease their attacks on our law enforcement and intelligence communities, and finally decide where their loyalty lies."
How does this fit into the wider picture?
The documents' release comes nine days after 12 Russians were charged with hacking Democratic officials in the 2016 US elections.
The charges form part of special counsel Robert Mueller's investigation into US intelligence findings that Russians conspired to tilt the election in Mr Trump's favour, and whether any of his campaign aides colluded. Mr Trump has labelled the investigation a "witch hunt".
So far, the inquiry has indicted 32 people - mostly Russian nationals in absentia.
Just days later, Mr Trump met his Russian counterpart, Vladimir Putin, at a summit in Helsinki and was asked by reporters whether he believed Russia had meddled in the 2016 election,
"President Putin says it's not Russia. I don't see any reason why it would be," he replied, appearing to back the Russians over his own agencies.
The next day, following widespread outrage, he read a pre-prepared statement, saying he actually meant "wouldn't".
On Thursday, the White House announced it had invited Mr Putin to Washington in October.