George Papadopoulos, a former campaign adviser to President Trump ensnared in the Russia probe, made his first appearance before Congress on Thursday to testify about his role in the events that ultimately led to the special counsel investigation into Russian meddling and possible collusion with the president's campaign.

Papadopoulos, who was convicted and sentenced in September to 14 days in jail for lying to the FBI, is the latest in a string of key figures to speak behind closed doors to congressional investigators looking into the FBI and the Justice Department's handling of the Russia probe. Republicans, many of whom allege agents acted improperly in launching the predecessor to the special counsel's investigation, described Papadopoulos as a critical component.

Following Papadopoulos' testimony, Rep. Mark Meadows, R-N.C., said "he was a very cooperative witness, very transparent, offering all of his emails, all of his communication. Certainly not the resume of someone who colluded or wants to hide anything from the American people.”

Meadows continued, "I think what’s troubling for me is that today, there is even a greater confirmation in my mind that there was inappropriate behavior on behalf of the FBI and the DOJ as they initiated this investigation," he continued.

The 31-year-old Papadopoulos was the first person to plead guilty in Special Counsel Robert Mueller’s probe into any ties between Russian operatives and the Trump campaign. According to the indictment, Papadopoulos was told by a Maltese professor in April 2016 that Russian officials had “dirt” on Hillary Clinton in the form of “thousands of emails.”

He allegedly later attempted to use his links to the professor and Russian nationals to attempt to set up a meeting between Trump and Russian President Vladimir Putin. He admitted to lying about those contacts to Mueller’s team, which investigators said led them to miss a chance to interview the professor, Joseph Mifsud.

Papadopoulos also told an Australian diplomat in Britain that Russia had dirt on Clinton during a night of drinking, the New York Times reported. Later, Australian officials would inform their American counterparts about Papadopoulos, according to the newspaper.

That meeting was part of the catalyst behind the FBI’s decision to begin investigating Russian interference in the 2016 election, according to the New York Times.

Republicans on the House Judiciary and Oversight Committees said they hoped Papadopoulos’ testimony would shed light on the origin of the FBI’s investigation into ties between Trump and Russia.

The Papadopoulos intelligence helped the FBI secure the surveillance warrant for ex-Trump campaign aide Carter Page in October 2016. But on Fox News’ “Sunday Morning Futures” with Maria Bartiromo, Rep. John Ratcliffe claimed “material facts” were withheld from the FISA court.

Meadows also tweeted that "the FBI omitted ... exculpatory information from the application," citing a report that Papadopoulos told the FBI there was no collusion.

Democrats, on the other hand, blasted Papadopoulos' credibility, saying he was already convicted of lying and calling the whole investigation “a sideshow.”

"Something everyone knows is that he’s been convicted of lying to the FBI,” Rep. Jaime Raskin, D-Md., said. “So you have to take his testimony for what it’s worth.”

Raskin added: “I see this whole thing as a footnote to a sideshow to a wild goose chase.”

Since he was sentenced to two weeks in prison, Papadopoulos has been eager to talk publicly. He has spent many nights on Twitter, along with his wife, venting anger with the FBI and implying that he was set up in the investigation.

“I think I was set up by Western intelligence. I think most of these meetings that I had were all orchestrated; I don’t know by who,” Papadopoulos said in an interview Saturday on Fox News.

Papadopoulos has also said he wants to talk to the Senate Intelligence Committee, which is investigating Russian interference in the 2016 election. A House Intelligence Committee investigation wrapped up earlier this year, with Republicans saying that there was no evidence of collusion between Russia and Trump's campaign.

Fox News’ Kevin Kirby, Adam Shaw and The Associated Press contributed to this report.