WASHINGTON (REUTERS) - George Papadopoulos has pleaded guilty to lying to FBI agents, the federal special counsel said on Monday (Oct 30), becoming the third adviser to President Donald Trump's campaign to face criminal charges in its investigation.

Papadopolous is an international energy lawyer who was on Trump's advisory team during the 2016 presidential campaign. The Chicago man pleaded guilty on Oct 5 in a case unsealed on Monday, the office of Special Counsel Robert Mueller said in a statement.

The court document said Papadopolous made false statements to the Federal Bureau of Investigation shortly after Trump's inauguration on Jan 20, when the law enforcement agency had an open investigation into Russian government efforts to interfere in the 2016 US presidential campaign.

The special counsel said Papadopoulos told FBI agents he had been in contact with an unnamed foreign "professor" who claimed to have "dirt" on Democratic presidential candidate Hillary Clinton in the form of "thousands of emails," and that Papadopolous claimed such contacts occurred before he joined Trump's campaign.

However, the prosecutor said Papadopolous in fact did not meet the professor until after he joined Trump's campaign.