The Pentagon said Thursday it is investigating President Donald Trump's former national security advisor Michael Flynn over payments he received from Russian government-linked firms.
The Department of Defense said it was probing whether the retired lieutenant general failed to obtain required prior approval before accepting money from foreign governments.
Flynn was specifically warned when he retired in 2014 that he had to get advance permission before accepting consulting fees, salaries, gifts, or travel expenses from a foreign government.
Flynn, the former defense intelligence chief, was fired by the White House after 24 days as Trump's top national security aide over his communications with Russia's US ambassador, amid intense concern that Moscow had directed a concerted effort to interfere in last year's presidential election.
But the Pentagon investigation, launched in early April, appears focused on the more than $33,000 dollars Flynn earned to attend an RT television gala in December 2015, where he sat at a table with Russian President Vladimir Putin.
It could also extend to the $530,000 Flynn was paid to lobby for Turkey during last year's presidential election campaign.
Such payments possibly violate Defense Department and US constitutional prohibitions on retired military personnel conducting activities involving foreign governments without advance clearance. Flynn ended a 33-year army career as director of the Defense Intelligence Agency during 2012-2014.
The Flynn probe adds to a number of investigations in Congress and the Department of Justice relating to Russian interference in the 2016 election, which US intelligence says was designed to boost Trump's prospects to win the presidency.
The Pentagon confirmed its investigation after the House Oversight Committee released a 2014 letter the DIA sent to Flynn advising specifically that he needed advance permission from Congress before accepting any payments from foreign governments.
Flynn's lawyer has said Flynn briefed the DIA before and after his Russia trip. But on Thursday Elijah Cummings, the committee's vice chair, said the DIA informed the committee that there is no evidence that Flynn requested approval to take the money from RT, which the US says is an arm of the Russian government.
"We have no evidence, zilch," that Flynn sought any approval to accept foreign payments, Cummings said.
Cummings also lashed out at the White House for not providing any documentation on Flynn in response to the committee's request.
"I honestly do not understand why the White House is covering up for Michael Flynn ... after the president fired him," Cumming said.
"It makes the American people think White House has something to hide," he added.