Michael Flynn. Win McNamee/Getty Images Congressional lawmakers investigating former national security adviser Michael Flynn say he may have lied in a 2016 interview with US investigators who were conducting a background check for his security-clearance renewal.

Documents suggest that Flynn told those investigators that during his trip to Moscow in December 2015 — in which he was pictured dining next to Russian President Vladimir Putin — he was "paid by 'US companies.'"

"The actual source of the funds for General Flynn's trip was not a U.S. company, but the Russian media propaganda arm, RT," said a statement from Rep. Elijah Cummings, the ranking Democrat on the House oversight committee. RT, or Russia Today, is the Russian state media outlet.

Cummings sent a letter to the committee's chairman, Republican Rep. Jason Chaffetz, describing the findings and asking Chaffetz to subpoena the White House for documents on the matter.

The Trump administration and Flynn's lawyers have so far refused to provide documents to bipartisan congressional committees overseeing the investigation into ties between associates of President Donald Trump and Russian operatives and government officials.

Flynn, through his attorneys on Monday, said he would invoke his Fifth Amendment right and decline a subpoena issued by the Senate Intelligence Committee for documents related to his interactions with Russian officials from June 2015 to January 2017.

Sens. Richard Burr and Mark Warner, the chair and ranking member of the Senate Intelligence Committee, responded to that announcement in a statement:

"While we recognize General Flynn's constitutional right to invoke the Fifth Amendment, we are disappointed he has chosen to disregard the Committee's subpoena request for documents relevant and necessary to our investigation. We will vigorously pursue General Flynn's testimony and his production of any and all pertinent materials pursuant to the Committee's authorities."

Flynn's payments from Russian entities have long been seen as red flags, before and after he was appointed to Trump's Cabinet as the national security adviser. Flynn agreed to testify before the House and Senate intelligence committees in late March in exchange for immunity from prosecution, but neither committee has agreed to that so far.

Flynn's time at the White House lasted just 24 days. He was asked to resign February 13, with the White House saying he had misled Vice President Mike Pence about conversations he had with Russia's ambassador to the US, Sergey Kislyak, during the transition. But Trump continued to defend him, going as far as suggesting to FBI Director James Comey the next day to drop the investigation into Flynn's foreign contacts, according to a memo Comey wrote about the conversation cited by The New York Times. Trump fired Comey on May 9.

After Flynn was forced out of the Trump administration, the president continued to vigorously advocate and defend Flynn — and blamed the Obama administration for having initially vetted Flynn for his brief Obama-era tenure as chief of the Defense Intelligence Agency.