WASHINGTON (AP) — A former Donald Trump campaign aide described by the White House as a low-level volunteer was thrust Monday into the center of special counsel Robert Mueller's investigation, providing key evidence in the first criminal case connecting Trump's team to alleged intermediaries for Russia's government.

George Papadopoulos was approached by people claiming ties to Russia and offering "dirt" on Hillary Clinton in the form of thousands of emails, according to court documents unsealed. Papadopoulos pleaded guilty to lying to FBI agents about the conversations and has been cooperating with investigators, the documents said.

Papadopoulos' guilty plea and the possibility that he's working with Mueller's team came as an unexpected twist in the mounting drama surrounding the criminal probe. A separate welter of charges Mueller announced Monday against Trump's ex-campaign chairman Paul Manafort and his longtime aide Rick Gates do not appear directly related to their work for Trump.

But Papadopoulos' case cuts close to the central question of Mueller's investigation: Did Russia try to sway the election? Did Trump's campaign know?

"The Russians had emails of Clinton," Papadopoulos was told by an unnamed Russian professor during a breakfast meeting at a London hotel in April. U.S. investigators said that the following day, Papadopoulos then emailed a Trump campaign policy adviser, "Have some interesting messages coming in from Moscow about a trip when the time is right."

Papadopoulos was arrested in July and has been interviewed repeatedly by authorities, the filing said. After entering his guilty plea he was ordered not to contact other Trump officials and prohibited from foreign travel. In one of the unsealed files, an FBI agent working for Mueller bluntly hinted that more former Trump associates could soon be questioned.

Papadopoulos' lawyer, Thomas M. Breen, based in Chicago, declined to comment on the guilty plea but noted that "we will have the opportunity to comment on George's involvement when called upon by the court at a later date. We look forward to telling all of the details of George's story at that time."

The incident echoes elements of a June 2016 meeting involving Donald Trump Jr. and other campaign officials at Trump Tower. The president's son organized that sit down with a Russian lawyer who was offering negative information about Clinton.

The White House immediately cast Papadopoulos as a mere volunteer with little influence during last year's campaign. White House spokeswoman Sarah Huckabee Sanders said his role was "extremely limited" and that "no activity was ever done in an official capacity on behalf of the campaign."

Trump named Papadopoulos to his foreign policy advisory council in March 2016, among a short list of experts amid growing public pressure on Trump to demonstrate he had a bench of foreign policy expertise.

During a meeting with the Washington Post editorial board, Trump called Papadopoulos an "excellent guy." He was named along with retired U.S. Army Lt. Gen. Keith Kellogg, former Bush administration inspector general Joseph Schmitz, international affairs professor Walid Phares and energy executive Carter Page, whose ties to Russian interests have also been scrutinized by congressional inquiries.

On March 31, 2016, Trump tweeted a photo of his meeting with the advisory board, with Papadopoulos among a handful of advisers at the president's table. In his plea filing, Papadopolous admitted that he told Trump and other top campaign national security officials during that meeting, that he had made contact with intermediaries for Russia who said they could set up a meeting between Trump and Russian President Vladimir Putin.

The statement does not describe Trump's reaction, but one meeting attendee recalled that it was not well received by Attorney General Jeff Sessions, then a senator and close adviser to the campaign who was also in attendance.

Sessions quickly dismissed the idea and said he'd prefer that no one ever speak about it, according to the attendee, who spoke on the condition of anonymity to share the private conversation.

The court filings recount Papadopoulos' meetings abroad starting in March 2016, after he'd been named to Trump's board. Papadopoulos initially told the investigators the meetings came before, and later admitted that was untrue. Papadopoulos also deleted a Facebook post about his travels, the documents said.

The court filings say he met first with the unnamed Russian professor who boasted of damaging emails and then later with an unnamed Russian woman, who claimed to be related to Putin and sought to arrange a meeting between Trump and the Russian leader. The unnamed professor also introduced Papadopoulos to a third unnamed person who claimed he had connections to the Russian Ministry of Foreign Affairs. The two men then exchanged emails about a possible meeting between Trump campaign aides and Russian government officials.

The court records didn't specify which emails the Russian claimed to have.

The timing of the new disclosures about Clinton emails may be significant because the scope of the Kremlin's efforts to hack Clinton's campaign and the Democratic National Committee were just beginning to be understood by March 2016, weeks before Papadopoulos was told of damaging emails.

It's unclear how frequently Papadopoulos was in contact with the campaign officials. Sanders initially said the foreign policy advisory board convened only once, but the White House later clarified she was speaking only of official meetings with Trump in attendance. An official involved with the group said the group met on a monthly basis throughout the spring and summer for a total of about six meetings.

Papadopoulos, who was based in London at the time, did not attend them all, but he did attend a dinner meeting of the advisers in late June at the Capitol Hill Club in Washington, along with Sessions.

The special counsel's statement noted that in an email exchange about Papadopoulos' contacts last year, two unnamed Trump campaign officials sought to protect Trump from any overt involvement.

"We need someone to communicate that DT is not doing these trips. It should be someone low level in the campaign so as not to send any signal," one of the campaign aides said. An email independently obtained by The Associated Press shows that the comment was sent by Trump's former campaign chairman Manafort to his long-time aide Gates on May 21, 2016.

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Associated Press writer Tom LoBianco contributed to this report.