CHICAGO (CBS) — President Donald Trump on Tuesday slammed a former campaign aide who became the first to plead guilty in special prosecutor Robert Mueller’s Russia probe, saying George Papadopoulos “has already proven to be a liar.”

Papadopoulos, who grew up in the Chicago area, was one of the first three people charged in connection to Mueller’s investigation of the Trump campaign’s ties to Russia, and the foreign power’s meddling in the 2016 election.

Former Trump campaign chairman Paul Manafort and his business associate Rick Gates also have been indicted on tax fraud, money laundering, and conspiracy charges that appear not directly related to their work on the campaign.

Papadopoulos has pleaded guilty to lying to federal agents working for special counsel Robert Mueller as part of his probe into possible coordination between the Trump campaign and Russia.

In a pair of tweets Tuesday morning, President Trump said Papadopoulos “has already proven to be a liar” and sought to deflect attention to the DNC, which contributed funding for research that culminated in the infamous dossier alleging Trump’s connections to Russia.

A former British spy, Christopher Steele, complied explosive allegations against Trump in a 35-page dossier for Fusion GPS during the 2016 campaign. The dossier alleged connections between then-presidential candidate Donald Trump and the Kremlin. Some of the information in the dossier is now under investigation by Mueller. According to the Associated Press, Steele met with investigators earlier this month.

Trump’s description of Papadopoulos as a “low level volunteer” and a “liar” is a stark contrast to his assessment during the 2016 campaign. In an interview with the Washington Post editorial board, then candidate Trump called Papadopoulas an “excellent guy” while naming members of his foreign policy team.

“George Papadopoulos, he’s an energy and oil consultant, excellent guy,” Trump said.

Papadopoulos, a graduate of Niles West High School and DePaul University, pleaded guilty on Oct. 5 to one count of making false statements to federal investigators. He admitted to lying to FBI agents about the nature of his interactions with “foreign nationals” who he thought had close connections to senior Russian government officials.

The plea was unsealed Monday.

Papadopoulos is the first person to face criminal charges that cite interactions between Trump campaign associates and Russian intermediaries during the 2016 presidential campaign.

Former federal prosecutor Jeffrey Cramer, who now is managing director of consulting firm Berkeley Research Group, said Papadapoulos’ plea signals he’s cooperating with the investigation.

“They’re clearly looking at connections, or relationships, or meetings that happened between the Trump administration and Russian representatives; and certainly on that level, that question, this individual played a role,” he said.

White House press secretary Sarah Huckabee Sanders downplayed Papadopoulos’ role within President Trump’s campaign, referring to him as a “volunteer.”

“He was a volunteer member of an advisory council that met one time,” Sanders told reporters Monday afternoon during the White House press briefing.

Papadopoulos, who is 30 years old, joined the Trump campaign in early March 2016 as a foreign adviser.

Sanders claimed that Papadopoulos was unpaid and asserted that his activities were considered “unofficial.”

“It has nothing to do with the campaign’s activities,” she said of Papadopoulos’ plea.

Sanders emphasized that she’s “not sure that the president recalls specific details” of a March 31 national security team meeting at Trump International Hotel in Washington, D.C. in which Papadopoulos was present.

“He asked to do things, he was basically pushed back or not responded to in any way,” Sanders said of Papadopoulos’ efforts within the campaign, which included sending out emails that offered to set up “a meeting between us and the Russian leadership to discuss U.S.-Russia ties under President Trump.”

According to court documents, Papadopoulos lied about his meetings with a Russian professor who had ties to the Russian government and who claimed to have “dirt” on Hillary Clinton–in the form of thousands of emails.  He also lied about his contacts with a female Russian foreign national and his efforts to arrange a meeting between Russian officials and the Trump campaign, according to court filings.

At one meeting with the Russian professor in London, the female Russian national was introduced to Papadopoulos as a relative of Russian president Vladimir Putin with “connections to senior Russian government officials,” according to court documents. In a subsquent email to unnamed Trump campaign advisers, Papadopoulos refered the the woman as “Putin’s niece.”

That meeting took place on March 24, just a few weeks after Papadopoulos was named as a foreign policy aide to the Trump campaign. His primary role, according to investigators, was to develop relationships between Russian officials and the Trump campaign.

On March 31, according to court documents, Papadopoulos attended a national security briefing in Washington with President Trump and senior campaign officials. At the meeting, Papadopoulos told the group that he had connections that would be able to arrange a meeting between then-candidate Trump and Putin.

On the same day, the Trump campaign posted a photo of the meeting on Instagram. Papadopolous is seated at the center of the conference table.

instagram Trump Slams Former Aide Who Pleaded Guilty In Russia Probe As Liar

Meeting held March 31 with then candidate Trump and campaign officials. (Credit: Instagram)

Over the course of the next several weeks, emails were exhanged about developing further relations and meetings between Russia and the Trump campaign.

In one of them, the female Russian national stated: ” I have already alerted my personal links to our conversation and your request. As mentioned we are all very excited by the possibility of a good relationship with Mr. Trump. The Russian Federation would love to welcome him once his candidature would be officially announced.”

Into the summer, Papadopoulos worked to arrange his own visit to Russia on behalf of the campaign. That meeting did not take place.

Trump aides have said he played a limited role in the campaign and no access to the president.

Papadopolous’ attorneys, in a statement, said: “It is in the best interest of our client, George Papadopoulos, that we refrain from commenting on George’s case.

“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.”