Trump-Russia investigators are looking into an uncorroborated letter that claims George Papadopoulos was pursuing a business deal with Russians "which would result in large financial gains for himself" and President Donald Trump after the 2016 election.

The letter is being examined by both congressional and FBI investigators, and two US officials told The Atlantic that the FBI is taking the letter's claims "very seriously."

Papadopoulos began serving out his two-week prison sentence for lying to the FBI this week.

But the revelation that officials are probing the letter indicates Papadopoulos is still a significant player in ongoing investigations into whether Trump associates colluded with Moscow during the 2016 election.

FBI and congressional investigators are looking into a new and uncorroborated claim that the former Trump campaign aide George Papadopoulos said he was pursuing a business deal with Russians "which would result in large financial gains for himself" and President Donald Trump, The Atlantic reported.

A Democratic source on the House Intelligence Committee confirmed to INSIDER that the letter was sent to ranking member Adam Schiff's office earlier this month from someone who claims to have been close to Papadopoulos in late 2016 and early 2017. Two US officials also told The Atlantic that federal authorities are investigating the letter and taking its claims "very seriously."

Papadopoulos pleaded guilty last year to one count of lying to the FBI and began serving his two-week prison sentence on November 26. But the revelation that authorities are probing the letter shows Papadopoulos is still a significant figure of interest in ongoing investigations into whether Trump associates colluded with Moscow during the 2016 election.

Papadopoulos denies having any financial links to Russia. But in a court filing announcing his guilty plea last year, prosecutors laid out multiple attempts by Papadopoulos to set up meetings between top campaign officials and Russians during the election.

Read more: George Papadopoulos tweets offer at Comey: Testify publicly and I’ll withdraw my request for immunity

Former Trump campaign adviser George Papadopoulos arrives at a closed-door hearing before the House Judiciary and Oversight Committee October 25, 2018 at Rayburn House Office Building on Capitol in Washington, DC. Alex Wong/Getty Images

Papadopoulos told the FBI that his outreach to the Russia-linked foreign nationals occurred before he joined the campaign. But his first interaction with an "overseas professor" with ties to high-level Russian officials occurred on March 14, 2016, weeks after he joined the campaign.

That professor, Joseph Mifsud, told Papadopoulos just over a month later that the Russians had dirt on Hillary Clinton that came in the form of "thousands of emails," according to the charging document.

Papadopoulos later told the top Australian diplomat Alexander Downer about the claim that Russia had kompromat on Clinton. That conversation, which Downer relayed to US officials, is what prompted the FBI to launch the Russia investigation.

Papadopoulos expressed remorse for lying to the FBI about his Russia connections during his sentencing hearing in September. But he has since adopted a drastically different tone by claiming in recent months that he was entrapped by the FBI and that his interactions with Mifsud were part of an elaborate set-up by British intelligence.

According to The Atlantic, Papadopoulos' baseless claims were what prompted the unnamed author of the letter to come forward. The person reportedly said they were willing to take a polygraph test "to prove that I am being truthful" and added that they decided to send the letter after observing Papadopoulos "become increasingly hostile towards those who are investigating him and his associates."

The individual also reportedly said Papadopoulos said in late 2016 that "Greek Orthodox leaders" and their Russian counterparts were "playing an important role" in Papadopoulos' interactions with the Russians.

Papadopoulos has met and interacted with several Greek officials, The Atlantic reported, most notably the Greek defense minister Pannos Kammenos. Kammenos' department was described to BuzzFeed News by a NATO official earlier this year as having been compromised by Russian intelligence.