At the same time, documents released by the top Democrat on a House oversight committee showed Flynn was warned by authorities after he retired from the military in 2014 not to take foreign government-sourced money without "advance approval" from the Pentagon.
Flynn, the former defense intelligence chief, was sacked by the White House after 24 days as Trump's top national security aide over his communications with Russia's U.S. ambassador, amid intense concern that Moscow had directed a concerted effort to interfere in last year's presidential election.
Flynn was ousted from his role as President Donald Trump's national security adviser after reports surfaced of him misleading Vice President Mike Pence about communications between himself and Russian ambassador Sergey Kislyak.
"This office has initiated an investigation to determine whether Lieutenant General (LTG) Flynn, U.S. Army (Retired) failed to obtain required approval prior to receiving any emolument from a foreign government".
Investigations intensified into Flynn on Thursday as the Pentagon inspector general's office joined lawmakers in probing payments he accepted from foreign sources including a Russian state-sponsored TV network.
"The Pentagon's warning to General Flynn was bold, italicized and could not have been clearer", Cummings said.
Flynn was reminded when he retired that accepting unauthorized payments from foreign governments was prohibited, according to the DIA letter. Consent is provided by Congress under 37 U.S.C. 908, which requires advance approval from the relevant service secretary and the Secretary of State before accepting employment, consulting fees, gifts, travel expenses, honoraria, or salary from a foreign government.
Rep. Elijah Cummings, the ranking Democrat on the House oversight committee, said earlier this week that while Flynn's negligence on his security clearance forms could be punished by up to five years in prison, that decision was not up to the committee.
An attorney for Flynn did not immediately respond to a request for comment.
"DIA did not locate any records referring to LTG Flynn's receipt of money from a foreign source..." Flynn has previously disclosed he got between $50,000 and $100,000 as part of his personal stake in $530,000 that his company, Flynn Intel Group, received for consulting work previous year for a Turkish businessman.
Flynn subsequently accepted more than $500,000 from a Russian government-owned broadcasting company and from a lobbying company representing Turkey, according to the committee.
"We have no evidence, zilch", that Flynn sought any approval to accept foreign payments, Cummings said. He also wants the Army to disclose any other investigations it's launched since 2010 into similar legal violations.
Flynn is also the subject of congressional probes into Russian interference in the 2016 presidential election that United States intelligence agencies have said was meant to sway the vote in favor of Trump, the Republican candidate, over his Democratic rival, Hillary Clinton.
"I see no evidence or no data to support the notion that Gen. Flynn complied with the law", Rep. Jason Chaffetz, the Republican chairman of the House oversight committee, said on Tuesday.
"There is obviously a paper trail that the White House does not want our committee to follow", the Democrats wrote in the letter.
The press secretary specified that his goal was not to criticize the decisions taken in the past, but to explain how the process works and who adjudicates a security clearance.
"Why would you rerun a background check on someone who was the head of the Defence Intelligence Agency that had and did maintain a high-level security clearance?" he said.