The investigation was revealed by House Democrats on Thursday.
Flynn, a former Army general, received the payment of $68,000 in 2015 for attending the 10-anniversary celebration of Russia Today, the Kremlin-owned media company.
He allegedly did not cite this payment on his SF-86, a document included in the screening process for a security clearance. Flynn's lawyer has denied any wrongdoing, saying Flynn "briefed the Defense Intelligence Agency, a component agency of the Department of Defense, extensively regarding the RT speaking event trip both before and after the trip."
Flynn resigned from his position in the Trump administration after just a few weeks when it emerged he had misled Vice President Mike Pence about conversations he had with Russian officials.
His argument seemed to be that since the Obama administration last issued Flynn a security clearance in 2016, the Trump administration wasn't responsible for vetting him further than that. (Flynn served in the Obama administration as the head of the Defense Intelligence Agency from 2012-2014 when he was eventually fired. He then worked as a foreign agent for Turkey during the 2016 campaign.)
"My only point is that when General Flynn came into the White House he had an active security clearance that was issued during the Obama administration, with all of the information that is being discussed that occurred in 2015," Spicer said, referring to the payment from Russia. Read full full disturbing story