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.