George Papadopoulos, a Trump-campaign adviser who pleaded guilty to lying to federal agents about his interactions with a Russia-linked professor in 2016, went to jail on Monday after fighting, and failing, to delay the start of his two-week prison sentence. But a letter now being investigated by the House Intelligence Committee and the FBI indicates that Papadopoulos is still in the crosshairs of investigators probing a potential conspiracy between the Trump campaign and Russia.

The letter, dated November 19 and obtained last week by The Atlantic, was sent to Democratic Representative Adam Schiff’s office by an individual who claims to have been close to Papadopoulos in late 2016 and early 2017. The letter was brought to the attention of Schiff and House Intelligence Committee staff, according to an aide who requested anonymity to discuss an ongoing investigation. The letter was also obtained by federal authorities, who are taking its claims “very seriously,” said two U.S. officials who also requested anonymity because of the sensitivities of the probe.

The statement makes a series of explosive but uncorroborated claims about Papadopoulos’s alleged coordination with Russians in the weeks following Trump’s election in November 2016, including that Papadopoulos said he was “doing a business deal with Russians which would result in large financial gains for himself and Mr. Trump.” The confidant—whose name The Atlantic is withholding on request but whose identity is known to congressional and federal investigators—stated a willingness to take a polygraph test “to prove that I am being truthful” and had come forward now after seeing Papadopoulos “become increasingly hostile towards those who are investigating him and his associates.” A lawyer for Papadopoulos declined to comment.

Read: A former Trump adviser pleads guilty to lying about his contacts with Russia.

If corroborated, the claims in the letter would add to an emerging portrait of Trump and his associates’ eagerness to strike backdoor deals with Russia even after the intelligence community concluded that Moscow had interfered in the 2016 election. (Trump’s son-in-law, Jared Kushner, tried to set up a “back channel” to Russia in the weeks after the election and met with the CEO of a sanctioned Russian bank during the transition period. Trump’s former national-security adviser, Michael Flynn, meanwhile, negotiated with the Russian ambassador about U.S. sanctions before Trump was inaugurated.)

Much of the attention in recent days has been focused on the former Trump-campaign chairman Paul Manafort and what the campaign knew about WikiLeaks’ plans to release stolen Democratic emails. But Papadopoulos remains one of the most important figures in the Russia investigation. He was ostensibly the first member of the Trump campaign to learn that the Russians had stolen emails that they planned to use against Hillary Clinton during the 2016 election. Rather than telling the FBI about the Russian “dirt” on Clinton, Papadopoulos continued trying to facilitate a meeting between Trump and Russian President Vladimir Putin as the campaign wore on. His disclosure to an Australian diplomat in May 2016 that Russia had dirt on Clinton is purportedly what triggered the FBI’s Russia investigation—Australian officials reported the comment to American law-enforcement authorities in July 2016, after WikiLeaks released the stolen DNC emails.

Federal and congressional investigators are now examining the letter to determine whether Papadopoulos’s ties to Russia were deeper than he has acknowledged, and whether he stayed in Trump’s orbit because of, rather than in spite of, those connections. The confidant who sent the letter to Schiff’s office last week claimed to have witnessed a phone call between Papadopoulos and Trump in December 2016, around the same time that Papadopoulos was allegedly boasting about the Russia deal and sending emails to Flynn and Trump’s campaign CEO, Steve Bannon. In one email, Flynn urged Papadopoulos to “stay in touch, and, at some point, we should get together.” Trump has called Papadopoulos “a coffee boy” who played no meaningful role on the campaign.

Papadopoulos, who has denied having any financial ties to Russia, has claimed in recent weeks that his contact with a shadowy overseas professor named Joseph Mifsud was a setup by Western intelligence agencies. Mifsud, who claimed to have high-level Kremlin contacts, told Papadopoulos in April 2016 that the Kremlin had dirt on Hillary Clinton in the form of thousands of emails—well before the Russian hacks on the Democratic National Committee or Hillary Clinton’s campaign chairman, John Podesta, were made public. Papadopoulos told the FBI that he learned of the Kremlin “dirt” before joining the Trump campaign, but that was a lie, according to prosecutors. He had already been a campaign adviser for well over a month by the time Mifsud told him about the stolen emails.

Read: George Papadopoulos hopes to fuel Republicans’ suspicions about the Russia probe.

Mifsud was also apparently eager to connect Papadopoulos with his current wife, Simona Mangiante. Mangiante told The Atlantic last month that she first heard about Papadopoulos and his work for the Trump campaign after starting a job at the London Centre of International Law Practice, where Mifsud was the “director for international strategic development,” in September 2016. Mifsud and his associate Nagi Idris told Mangiante over lunch that Papadopoulos, who worked at the London Centre briefly in the spring of 2016, would be visiting London soon, and that if Mangiante met him, she should “make sure” she said she liked Trump—or not discuss politics at all. Mangiante insists, however, that Mifsud never directly introduced her to Papadopoulos, whom she says she met on LinkedIn later that fall.

Mifsud may be only one part of the story of Papadopoulos’s connections to Russian nationals in 2016. According to the letter sent to Schiff last week, Papadopoulos revealed in late 2016 that “Greek Orthodox leaders” and their Russian counterparts were “playing an important role” in Papadopoulos’s collaboration with the Russians.

Papadopoulos’s contact with Greek officials in 2016 has been of some interest to those investigating a potential conspiracy between the Trump campaign and Russia. In a September interview with CNN, Papadopoulos acknowledged for the first time that he told Greece’s foreign minister about the Russian “dirt” on Clinton in May 2016 while visiting the country on a trip authorized by the Trump campaign. Putin was set to visit Greece the very next day, and the foreign minister “explained to me that where you are sitting right now, tomorrow Putin will be sitting there,” Papadopoulos told CNN, claiming that his disclosure about the “dirt” was “a nervous reaction” that he just “blurted out.”

Read: What is a ‘proactive cooperator’?

Throughout 2016, Papadopoulos made multiple trips to Greece and developed a working relationship with influential Greek officials while he was serving as a foreign-policy adviser on the Trump campaign. In addition to Foreign Minister Nikos Kotzias, Papadopoulos had meetings with the former president of Greece, Prokopis Pavlopoulos, and Ieronymos II, the archbishop of Athens and All Greece. Papadopoulos’s closest association with the Greek government, however, appears to have been with Greek Defense Minister Panos Kammenos, an outspoken supporter of Moscow with whom Papadopoulos met several times in 2016 and early 2017, including at Trump’s inauguration. In his congratulatory tweet celebrating Trump’s election victory, Kammenos noted Papadopoulos’s importance in maintaining U.S.-Greek relations. A NATO military-intelligence official told BuzzFeed News earlier this year that the Greek Ministry of Defense “is considered compromised by Russian intelligence.”

It remains to be seen whether Schiff, the incoming chairman of the House Intelligence Committee, will subpoena Papadopoulos to appear before the panel once the Democrats take control in January. Patrick Boland, a spokesman for Schiff, told The Atlantic that “at the appropriate time,” the congressman hopes to get “full answers on the range” of Papadopoulos’s “contacts with the Russians and their intermediaries.” Boland said that Schiff and his staff “evaluate all information brought to our attention and remain concerned about the conduct that formed the basis of Mr. Papadopoulos’s guilty plea, as well as his subsequent and apparently contradictory statements.” Papadopoulos pleaded guilty to making false statements to investigators about “the timing, extent, and nature of his relationships and interactions with certain foreign nationals whom he understood to have close connections with senior Russian government officials.”

Papadopoulos is not a stranger to the halls of Capitol Hill. He faced Congress for the first time in October, in a private hearing before the House Oversight and Judiciary Committees, for which he eagerly volunteered following his sentencing one month prior. “I didn’t want to have to expose the biggest political scandal in modern history,” Papadopoulos tweeted ahead of his testimony. “I was happy living on the Greek islands. But, guess life works in mysterious ways and I am happy I was called.” Despite promoting his unqualified willingness to testify before Congress, however, Papadopoulos asked the Senate Intelligence Committee for immunity before agreeing to testify. While the Senate is unlikely to accommodate him, the request itself may signal an awareness on Papadopoulos’s part—or his lawyers’—that he still faces significant legal exposure.