(CNN) Paul Manafort resigned as President Donald Trump's campaign manager six months ago. Now he's back in the spotlight, amid word from multiple sources that he and other senior Trump advisers were in regular communication during the campaign with Russians known to US intelligence -- claims that Manafort denies.

So who is Manafort and why is he under scrutiny?

A Republican strategist and longtime Washington operator, he joined Trump's campaign team last spring.

He was originally brought in to save Trump from a defeat at the Republican convention, should Trump have failed to win enough delegates to clinch a first-ballot nomination.

But his role grew to serve as Trump's connection to the GOP establishmen t, telling Republican elders that their presidential nominee would run a traditional campaign that would not imperil down-ballot candidates.

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In August, with just under three months to go until the presidential election, Manafort resigned amid questions over his campaign role and his extensive lobbying history overseas, particularly in Ukraine , where he represented pro-Russian interests.

Trump's son Eric told Fox News at the time that while Manafort had been instrumental in steering the campaign through the GOP convention, the former chairman's business past had begun to detract from his father's messages.

Manafort was replaced as campaign manager by Kellyanne Conway , now a top White House adviser. Breitbart News chief Steve Bannon was at the same time made campaign CEO. He's now Trump's chief strategist in the White House.

Manafort, a longtime K Street lobbyist and former Ronald Reagan operative, reportedly remained in Trump's orbit during the transition period but the Trump team said he was not part of the transition team.

With a history as a Republican adviser dating back to President Gerald Ford's 1976 campaign, he has a large network of Washington contacts.

Flynn fallout

Flynn and Manafort were among several high-level advisers to then-presidential nominee Donald Trump and were in regular communication during the campaign with Russians known to US intelligence, multiple current and former intelligence, law enforcement and administration officials told CNN.

CNN has reached out to the White House and Flynn for comment.

Donald Trump is flanked by then-campaign manager Paul Manafort as he prepares to accept the Republican party's nomination on July 20.

"That is 100% not true, at least as far as me," he said. "I cannot believe that they are including me in anything like that. I have not been involved in any of these activities."

Manafort said he did not know where US officials got the idea that he was in contact with suspected Russian operatives during the campaign but said he never spoke with any Russian officials during that time.

"I don't remember talking to any Russian officials, ever. Certainly during the time we're talking about," he said, calling the allegations "boggling."

"I have knowingly never talked to any intelligence official or anyone in Russia regarding anything of what's under investigation," he said. "I have never had any connection to (Russian President Vladimir) Putin or the Russian government before, during or after the campaign."

Business ties

Manafort said the FBI had not contacted him about the allegations and said he was not aware of any other Trump campaign officials or people close to Trump being in touch with Russians known to US intelligence.

Manafort, who has held business ties with Russian and Ukrainian individuals, also emphasized that his work for the Yanukovych government in Ukraine should not be interpreted as closeness to the Russians. He said he worked for Yanukovych during a time when Ukraine was "moving into the European orbit."