WASHINGTON — Russian intelligence services pursued myriad avenues to influence the Trump campaign in 2016, according to the Senate Intelligence Committee, but none was more important than the relationship between the campaign chairman Paul Manafort and a man who had been his friend and co-worker for years: a Russian intelligence officer named Konstantin V. Kilimnik.
Their link was “the single most direct tie between senior Trump campaign officials and the Russian intelligence services,” according to the fifth and final volume of the committee’s report on its bipartisan three-year investigation issued Tuesday.
While the committee was stymied in its efforts to fully plumb the relationship between the two men, investigators found enough there to declare that Mr. Manafort created “a grave counterintelligence threat” by sharing inside information about the presidential race with Mr. Kilimnik and the Russian and Ukrainian oligarchs whom he served.
The report portrayed Mr. Manafort as deeply compromised by years of business dealings with those oligarchs. Collectively, they had paid him tens of millions of dollars, lent him millions more and may also have owed him millions.
These complex financial entanglements apparently figured in Mr. Manafort’s decision to give Mr. Kilimnik inside campaign information, including confidential polling data and details of Mr. Trump’s campaign strategy. The report builds on other evidence suggesting that Mr. Manafort hoped that Mr. Kilimnik would open up lucrative business deals with the oligarchs in return or that they would consider the value of the information as its own form of payment.
The committee had little explanation for the connection between the two men, citing Mr. Manafort’s lies to federal authorities, coupled with the care the two men took to protect their communications, as roadblocks to learning more.
“What did the Russians do with all this information, how did they use it, did they use it?” Senator Mark Warner of Virginia, the committee’s top Democrat, asked in an interview on Tuesday. “Those are serious counterintelligence questions we may never get the full answer on.”
The report said Mr. Kilimnik was Mr. Manafort’s link to Oleg V. Deripaska, a Russian oligarch who is close to President Vladimir V. Putin of Russia and has acted “as a proxy for the Russian state and intelligence services” since at least 2004, when Mr. Manafort apparently met him.
Mr. Deripaska, who has worked to install pro-Kremlin governments around the globe, initially hired Mr. Manafort as a political consultant, the report said. A group of pro-Russia oligarchs in Ukraine later became the financiers of Mr. Manafort’s operations to help Viktor F. Yanukovych, a politician aligned with Russia, become Ukraine’s president.
Mr. Manafort recognized the Kremlin’s interests, the report said. “This model can greatly benefit the Putin government if employed at the correct levels with the appropriate commitments to success,” he wrote in a memo to Mr. Deripaska.
The report called Mr. Manafort’s efforts for the oligarch “in effect, influence work for the Russian government and its interests.”
For over a decade, the work made Mr. Manafort fabulously wealthy. At lunch after Mr. Yanukovych was elected president in 2010, the report said, the new Ukrainian leader “snapped his fingers” and gave Mr. Manafort a jar of caviar worth $30,000 to $40,000.
Despite questions about who was behind Mr. Kilimnik — both financially and politically — Mr. Manafort increasingly depended on him. But by 2014, the Ukraine work had dried up.
Mr. Yanukovych had been forced out as president after a popular uprising and fled to Russia. Mr. Manafort claimed the Ukrainian oligarchs had stiffed him out of millions for his work for Mr. Yanukovych. And Mr. Deripaska was trying to collect from Mr. Manafort for a failed private equity deal in Eastern Europe.
Now broke, Mr. Manafort volunteered to work for the Trump campaign, which hired him in March 2016. In a memo, Mr. Manafort offered to brief Mr. Deripaska on “this development with Trump.”
Mr. Manafort also speedily passed along the news of his new job to Mr. Kilimnik, who traveled to the United States specifically to meet him in May and again in August 2016. According to the report, Mr. Manafort was forthcoming: He briefed Mr. Kilimnik on Mr. Trump’s path to victory and his strategy to win in battleground states.
After he rose to campaign chairman, Mr. Manafort also instructed his deputy, Rick Gates, to periodically share confidential Trump campaign polling data with Mr. Kilimnik, including surveys showing what voters most disliked about Hillary Clinton, Mr. Trump’s Democratic opponent. Mr. Gates “understood that Kilimnik would share the information with Deripaska,” the report said.
The transfer of internal campaign data to a known Russian agent is “about as clear a coordination or cooperation between two entities as could be established,” said Senator Angus King, a Maine independent on the Senate Intelligence Committee who votes with Democrats.
The committee said it found evidence — redacted for national security reasons — that Mr. Kilimnik may have been involved in the covert effort by the Russian government to hack into the computer networks of Democratic organizations and funnel damaging emails to the rogue website WikiLeaks, which released them just before the election.
The report also cited but did not reveal information it said potentially links Mr. Manafort to that operation, which was by far Russia’s most significant effort to disrupt the American election.
Mr. Manafort was forced to resign from the Trump campaign in August 2016 amid a growing scandal over his work in Ukraine. He later told the F.B.I. that he had briefed Mr. Trump on his Ukraine work, but “did not go into detail because Trump was not interested.”
Even after he was ousted, Mr. Manafort stayed in touch with campaign officials and with Mr. Kilimnik, who believed Mr. Manafort could still influence the new administration’s foreign policy, the report said.
Together, the men also promoted the false, Kremlin-backed story that Ukraine, not Russia, interfered in the 2016 election. The report stated the similarities in their efforts suggested coordination.
Yet what exactly they might have said to each other remains a mystery. The pair used encryption applications such as Viber, Signal and WhatsApp or exchanged emails through “foldering,” a technique that allows people to see the messages without actually sending them. They would alert each other to check the “tea bag” or the “updated travel schedule” for a new message.
After he was convicted of orchestrating a financial fraud scheme, Mr. Manafort agreed to cooperate with federal prosecutors working for the special counsel, Robert S. Mueller III, who was investigating Russian interference in the election.
The prosecutors were especially eager to question him about Mr. Kilimnik, who was also indicted on charges of obstruction of justice but could not be extradited. Mr. Kilimnik has denied any ties to Russian intelligence services.
Prosecutors ultimately decided that Mr. Manafort was lying to them and pulled out of a plea agreement with him. He is now serving his seven-and-a-half year prison sentence at home because of the coronavirus pandemic.
After he was indicted, Mr. Manafort bought a pay-as-you-go phone, the report said. One primary purpose was to talk to Mr. Kilimnik.