Mr. Papadopoulos repeatedly promoted the idea of a “history making” meeting between Mr. Trump and Vladimir V. Putin, the Russian president. Senior campaign officials, however, said that Mr. Trump should not make the trip and leave it to someone “low level in the campaign so as not to send any signal,’’ according to an email cited in court documents.
Mr. Papadopoulos then proposed that he himself, perhaps with another campaign official, travel to Moscow to meet with the Russians.
“The trip proposed by defendant PAPADOPOULOS did not take place,” prosecutors wrote.
To grasp the significance of Monday’s developments, it helps to recall exactly how the Russian attack unfolded.
In September 2015, the F.B.I. made its first call to the Democratic National Committee to report evidence of Russian hackers inside the committee’s network. But for seven months, the word never got beyond an I.T. contractor, and the hackers apparently had the run of confidential emails and other files.
During that time, Mr. Trump was pressed to assemble a team of foreign policy advisers, a difficult task because he was shunned by many Republicans who had served in earlier administrations. In early March, Mr. Papadopoulos, who had been helping the beleaguered campaign of Dr. Ben Carson, offered his services to the Trump campaign.
Around March 6, documents say, a campaign supervisor – identified by a former Trump adviser as Sam Clovis – told Mr. Papadopoulos, then living in London, that “a principal foreign policy focus of the campaign was an improved relationship with Russia.”
A week later, traveling in Italy, Mr. Papadopoulos encountered a London-based professor of international relations, Joseph Mifsud, who claimed to have “substantial connections with Russian government officials.” (The court documents do not name Mr. Mifsud, but a Senate aide briefed on the case identified him as the professor in question.)
Unimpressed by Mr. Papadopoulos at first, Mr. Mifsud became far more interested when he learned that the young traveler was working for the Trump campaign. The two men met again in London on March 24, when the professor introduced Mr. Papadopoulos to a Russian woman he said was a relative of Mr. Putin with close ties to senior Russian officials.
The same week, Mr. Trump, visiting The Washington Post, was pressed to name his foreign policy team. He read out five names, most of them with modest or nonexistent public profiles – including Mr. Papadopoulos. Mr. Papadopoulos began emailing campaign officials about his new contacts with his “good friend” Mr. Mifsud and the Russian woman, whom he incorrectly believed was Mr. Putin’s niece, and the possibility of a Trump-Putin meeting.
On March 31, back in Washington, Mr. Papadopoulos met Mr. Trump for the first time at a gathering of his new foreign policy team at the candidate’s Washington hotel. According to the former Trump adviser who was there, and who spoke on condition of anonymity to avoid offending former colleagues, Mr. Papadopoulos spoke for a few minutes about his Russian contacts and the prospects for a meeting with the Russian president.
But several people in the room began to raise questions about the wisdom of a meeting with Mr. Putin, noting that Russia was under sanctions from the United States. Jeff Sessions, now attorney general and then a senator from Alabama who was counseling Mr. Trump on national security, “shut George down,” the adviser said. “He said, ‘We’re not going to do it’ and he added, ‘I’d prefer that nobody speak about this again.’”
But Mr. Papadopoulos was not deterred, the documents say, and he continued to communicate with Mr. Mifsud and the Russian woman about more contacts. The Russian woman wrote on April 11, “we are all very excited by the possibility of a good relationship with Mr. Trump.” Mr. Mifsud introduced Mr. Papadopoulos over email to a Moscow contact who said he had connections to the Russian foreign ministry. They spoke repeatedly over Skype about a possible Moscow trip, the documents say.
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On April 26 came a crucial meeting. At breakfast at a London hotel, Mr. Mifsud told Mr. Papadopoulos that he had just returned from Moscow, where he had “learned that the Russians had obtained ‘dirt’ on then-candidate Clinton.” Mr. Mifsud said he had been told the Russians had “thousands of emails.”
On May 4, the Russian contact with ties to the foreign ministry wrote to Mr. Papadopoulos and Mr. Mifsud, saying ministry officials were “open for cooperation.” Mr. Papadopoulos forwarded the message to a senior campaign official, asking whether the contacts were “something we want to move forward with.”
The court documents describe in detail how Mr. Papadopoulos continued to report to senior campaign officials on his efforts to arrange meetings with Russian officials, which The Washington Post reported on in August. But the documents do not say explicitly whether, and to whom, he passed on his most explosive discovery – that the Russians had what they considered compromising emails on Mr. Trump’s opponent.
J.D. Gordon, a former Pentagon official who worked for the Trump campaign as a national security adviser and helped arrange the March 31 foreign policy meeting, said he had known nothing about Mr. Papadopoulos’ discovery that Russia had obtained Democratic emails or of his prolonged pursuit of meetings with Russians.
“I was surprised to learn what George Papadopoulos was up to during the campaign,” Mr. Gordon said in a text message. “He obviously went to great lengths to go around me and Senator Sessions.”
Mr. Gordon said that such end-runs around normal channels are common in presidential campaigns. “It’s very hard to know what every single person is doing, especially since some folks deliberately go around the chain of command,” he said. “But George Papadopoulos obviously represents an extreme case.”
Prosecutors may have deliberately left salient details out of the documents filed in court to protect the continuing investigation. But what they did say portrays Mr. Papadopoulos as continuing for months to arrange meetings with Russian officials. As late as August 2016, Mr. Papadopoulos was advised by a campaign official, apparently Mr. Clovis, to travel to Moscow “if it is feasible.”
Instead of opening a new era in relations with Russia, Mr. Papadopoulos found himself caught up in the investigation of the Russian interference in the American election. The documents accuse him of lying to F.B.I. agents in two interviews, first on Jan. 27 and then on Feb. 16, when he “reiterated his purported willingness to cooperate with the F.B.I.’s investigation,” according to an affidavit filed by Robert M. Gibbs, an F.B.I. agent investigating the case.
On Feb. 17, the agent wrote, Mr. Papadopoulos shut down the Facebook account he had used since 2005, including to exchange messages with the intermediaries for Russia. Prosecutors considered that to be obstruction of justice, and it became part of the case that concluded Oct. 5 when Mr. Papadopoulos pleaded guilty to make a false statement to the F.B.I., which carries a maximum penalty of five years in prison.
An earlier version of this article misstated the legal status of George Papadopoulos. His guilty plea was unsealed on Monday; he was not indicted on Monday.