An advisor to President Donald Trump's campaign pleaded guilty on Oct. 5 to lying to FBI agents about when he met with Russian nationals to get "dirt" on Hillary Clinton, according to court filings released Monday.

The documents revealed that George Papadopoulos was arrested on July 27 upon arrival at Dulles International Airport from an undisclosed location.

According to the documents, shortly after Papadopoulos learned he would become a foreign policy advisor to the Trump campaign, he met in March 2016 with a Russian professor who claimed to have "dirt" on Clinton. Investigators say Papadopoulos had told them he met with the professor before he learned of his role in the campaign.

Ten days later, Papadopoulos met with a female Russian national who was introduced to him as a niece of Russian President Vladimir Putin with ties to senior Kremlin officials, investigators said.

Papadopoulos later learned, however, that the woman was in fact not related to Putin.

He told investigators that his correspondence with the woman was superficial when in fact the purpose of the contacts was to set up a meeting between Russian leaders and the Trump campaign, the court filings said.

The filings say Papadopoulos is cooperating with the U.S. government in its ongoing investigation of Russian attempts to interfere in the 2016 election.

In a Monday afternoon press briefing, White House press secretary Sarah Huckabee Sanders said Papadopoulos had an "extremely limited" role in the 2016 campaign. She said he was in a "volunteer position" and had not done work in an "official capacity on behalf of the campaign."

But investigators said a campaign supervisor had urged Papadopoulos to try to visit Russia for an off-the record meeting with officials.

Papadopoulos' attorneys declined to comment on the case.

"We will have the opportunity to comment on George's involvement when called upon by the Court at a later date," his legal representatives said. "We look forward to telling all of the details of George's story at that time."