Former Trump campaign foreign policy adviser George Papadopoulos began serving his two-week prison sentence on Monday after a judge rejected his last-minute bid to remain free.

Papadopoulos, 31, arrived at a minimum-security camp in Oxford, Wis., according to the Federal Bureau of Prisons.

Papadopoulos, the first campaign aide sentenced in Mueller's investigation, triggered the Russia investigation two years ago. He was sentenced in September for lying to the FBI about his interactions with Russian intermediaries during the 2016 presidential campaign.

He had sought a postponement of his prison term until an appeals court ruled in a separate case challenging the constitutionality of special counsel Robert Mueller's appointment.

But U.S. District Court Judge Randolph Moss said Papadopoulos had waited too long to contest his sentence.

George Papadopoulos, far left, is shown at a March 31, 2016, meeting of Donald Trump campaign officials, with Trump seated at the end of the table. (Donald Trump/Instagram)

The White House has said Papadopoulos, 31, was a low-level volunteer on the campaign. But he's been a central figure in the Russia investigation dating before Mueller's May 2017 appointment.

According to an indictment handed up this summer, Russian intelligence had stolen emails from Hillary Clinton's campaign and other Democratic groups by April 2016, the same month Papadopoulos was informed by a professor that Russian officials had told him they had "dirt" on Clinton in the form of "thousands of emails."

Contact with Russians

Papadopoulos later used his connections with the Maltese professor, Joseph Mifsud, and other Russian nationals in an attempt to broker a meeting between then-candidate Donald and Russian President Vladimir Putin.

Papadopoulos admitted last year to lying to the FBI about those contacts with Russians and Russian intermediaries.

In recent months, Papadopoulos has spent many nights posting on Twitter, venting anger about the FBI and insisting he was framed by the government. He also has offered to testify before the Senate intelligence committee, which is investigating Russian interference in the 2016 election, if he's granted immunity or other conditions.

"The truth will all be out. Not even a prison sentence can stop that momentum," Papadopoulos tweeted Sunday. "Looking forward to testifying publicly shortly after. The wool isn't going to be pulled over America's eyes forever."

Papadopoulos' sentence, issued by Moss on Sept. 7, was far less than the maximum six-month sentence sought by the government but more than the probation that Papadopoulos and his lawyers had asked for.

Moss at the time noted that many similar cases resulted in probation but said he imposed a sentence of incarceration partly to send a message to the public that people can't lie to the FBI.