May 22 (UPI) -- Former national security adviser Michael Flynn -- a critical figure in the United States' Russia inquiries -- denied investigators Monday by refusing to comply with a Senate subpoena.
Flynn was ordered to hand over documents related to the investigation, which seeks to determine if there was any collaboration between President Donald Trump's team and the Kremlin before the November election.
However, Flynn invoked his Fifth Amendment privilege and declined to furnish the panel with relevant documents in his possession.
"The context in which the committee has called for General Flynn's testimonial production of documents make clear that he has more than a reasonable apprehension that any testimony he provides could be used against him," Flynn's attorney wrote in a letter to the committee.
"Producing documents that fall within the subpoena's broad scope would be a testimonial act, insofar as it would confirm or deny the existence of such documents."
Seeking protection under the Fifth Amendment, which guarantees Americans the right against self-incrimination, allows Flynn to remain silent without admitting guilt.
His attorney's letter also cites an "escalating public frenzy" and the appointment of former FBI chief Robert Mueller as a special prosecutor in the case as factors in Flynn's decision.
The former adviser is party to three simultaneous U.S. investigations into the Russia affair at the Justice Department and in both chambers of Congress. While invoking the Fifth Amendment allows him to protect himself, it could also result in a criminal charge of contempt of Congress -- a possibility acknowledged Monday by Virginia Sen. Mark Warner, the committee's ranking Democrat.
"We have to find out whether we have the ability to either hold Gen. Flynn in contempt or whether it's just Fifth Amendment," he told CNN. "I've got to get the legal answer to that first."
One source told NBC News that it would be "imprudent" for Flynn to hand over any documents, which the panel has been seeking since last month.
Also Monday, Rep. Elijah Cummings, D-Md., said documents already in the House Oversight Committee's possession appear to indicate that Flynn may have lied to investigators last year when they were conducting a background check for his top-level security clearance.
Cummings said the documents show that Flynn told investigators that U.S. companies had paid for a 2015 trip to Moscow, where he delivered a speech at an event honoring the Russian news outlet RT. However, Cummings said, it appears that RT directly paid for Flynn's airfare, lodging and other expenses -- as well as his $45,000 speaking fee.
It is a crime to falsify or conceal information on a security clearance application.
"We need to know what the president, vice president, White House counsel and other top officials knew about General Flynn -- and when they knew it," Cummings wrote in a letter to House Oversight Committee Chairman Jason Chaffetz, R-Utah.
Flynn was fired by Trump 24 days into his stint as national security adviser over his contacts with Russian diplomat Sergei Kislyak in December -- discussions the president said he failed to disclose to Vice President Mike Pence.
Flynn is a central figure in the Russia investigation because of his known contacts with Kislyak and the top-level role he subsequently filled in the Trump administration.
The U.S. intelligence community believes Moscow, with the approval of President Vladimir Putin, attempted to sway the election in Trump's favor last year. All three investigations are looking into that same prospect, as well as the possibility of cooperation between Trump's campaign and the Kremlin to achieve that goal.
Trump has repeatedly denied there was any collusion between his team and Russian officials.