In this July 18, 2016, file photo, Trump campaign chairman Paul Manafort walks around the convention floor before the opening session of the Republican National Convention in Cleveland. AP Photo/Carolyn Kaster, File Retired Gen. Michael Flynn resigned from his position as President Donald Trump's national security adviser on Monday amid reports that he discussed sensitive national-security information with Russia's ambassador to the US while he was still a private citizen.

Flynn, a loyal surrogate for Trump during his presidential campaign, is now the third Trump adviser — and second top Trump official — to resign over his ties to Russia throughout the course of Trump's campaign and nascent presidency.

Paul Manafort, Trump's former campaign manager, was first. Carter Page, an early foreign policy adviser to the Trump campaign, subsequently took a leave of absence from the campaign amid scrutiny over his dealings with Russia.

All three are mentioned in an unsubstantiated dossier alleging serious misconduct and conspiracy between the Trump campaign and Russia's government. The document's findings were presented by top US intelligence officials to President Donald Trump and senior lawmakers last month.

The White House has dismissed the dossier as fiction, and some of the facts and assertions it includes have indeed been proven wrong. But some of the dossier's material has been corroborated by US intelligence officials, CNN reported last week.

Intelligence officials began looking into potential contact between Trump's team and Russian officials when Russian President Vladimir Putin decided not to retaliate against sanctions introduced by President Barack Obama in December, The Washington Post reported last week.

Officials discovered that Flynn called the Russian ambassador to the US, Sergey Kislyak, the day Obama imposed the new penalties and gave him "the impression that the sanctions would be revisited at a later time." The officials concluded that Flynn had spoken to Kislyak several times before Trump was sworn in.

According to the unverified dossier, the Kremlin began cultivating Flynn in 2015, when it funded his trip to Moscow and paid him to speak at a gala celebrating the 10th anniversary of the state-sponsored news agency Russia Today. Top Democrats are now calling for an investigation into whether Flynn violated the Emoluments Clause of the Constitution by accepting money from a foreign country.

Manafort, Trump's former campaign manager who advised a pro-Russia political party in Ukraine for nearly a decade, resigned five days after The New York Times reported that the party had earmarked $12.7 million for Manafort for his work between 2007-2012.

The dossier claims that Manafort "managed" the communication between the Trump campaign and the Kremlin, and was receiving "kickback payments" from deposed Ukrainian President Viktor Yanukovych throughout 2016. Yanukovych was ousted in 2014 and has lived in Russia under the protection of the Kremlin since.

"When campaign chairman and NSA [national security adviser] both resign over Russia ties there is more," Ben Rhodes, a former top national-security adviser to Obama, tweeted on Tuesday. "Manafort and Flynn had nothing in common except Russia and Trump."

As happened with Manafort, Page's role within the Trump campaign changed after news of his Russia connections became public. Page, whom Trump named as early foreign policy adviser to the campaign, served as an adviser "on key transactions" for Russia's state-owned energy giant Gazprom before setting up his own energy investment fund, Global Energy Capital, with former Gazprom executive Sergei Yatesenko.

Page traveled Moscow for three days in mid-July, when he gave a speech at the New Economic School and, according to Yahoo, met with the sanctioned CEO of Russia's state oil company, Igor Sechin. The dossier, meanwhile, alleged that Sechin offered Page the brokerage of the sale of a 19% stake in Rosneft if he pressured Trump to lift sanctions on Russia.

Page denied meeting with any sanctioned individuals during his trip, but he took "a leave of absence" from the Trump campaign shortly after the Yahoo report was published. The Trump campaign subsequently distanced itself from Page, claiming it never worked with him.

Page's extensive business ties to state-owned Russian companies were investigated by a counterintelligence task force set up last year by the CIA, according to several media reports. The investigation, which is reportedly ongoing, has examined whether Russia was funneling money into Trump's presidential campaign — and, if it was, who was serving as the liaison between the Trump team and the Kremlin.