WASHINGTON (AP) — President Trump's former national security adviser, Michael Flynn, was warned by authorities when he retired from the military in 2014 not to take foreign government-sourced money without "advance approval" from the Pentagon, according to documents released Thursday by the ranking Democrat on a House oversight committee.

Flynn, a former U.S. Army lieutenant general and Defense Intelligence Agency chief, was later paid tens of thousands of dollars for his work on behalf of foreign interests, including Russia's state-sponsored RT television network and a Turkish-owned company linked to Turkey's government.

Rep. Elijah Cummings, D-Md., also said the Pentagon's acting inspector general has launched an inquiry into whether those payments qualify as coming from foreign governments and whether Flynn properly informed military authorities about them.

Both Cummings and Utah Republican Jason Chaffetz, the chairman of the Committee on Oversight and Government Reform, have previously said they planned to ask the Army to rule on whether Flynn properly informed and asked permission for the payments from Russian and Turkish entities.

A key document released Thursday by Cummings showed that Flynn was warned by a Defense Intelligence Agency official in October 2014 that he would need clearance from the agency before he could accept any earnings linked to foreign governments. Flynn was explicitly told in the document that the U.S. Constitution's emoluments provision prohibits any monetary payments or gifts "from a foreign government unless congressional consent is first obtained." The letter explained that such "advance approval" would need to come "from the relevant service secretary."

The committee's leaders reported earlier this week that they found no evidence that Flynn asked for permission for the payments he received or informed the military that he had accepted them. Army spokeswoman Cynthia O. Smith Thursday also said "we have found no records of LTG(R) Flynn requesting permission from the Army for foreign employment."

In comments to the AP, Chaffetz said that Flynn "had an obligation to seek approval to take money from a foreign government. We found no evidence that he did that." Chaffetz, however, did not join Cummings at the Thursday news conference, unlike their bipartisan appearance earlier in the week announcing the results of the committee's inquiry into Flynn's dealings with authorities before and after his foreign earnings.

The committee's inquiry is one of several congressional investigations into Flynn's contacts with foreign officials before and during his brief stint as Trump's top national security aide. Trump fired Flynn for failing to inform senior administration officials about his contacts with Russian officials — contacts that are being examined as part of the wider inquiries into Russian meddling in the 2016 presidential campaign.

"These documents raise grave questions about why General Flynn concealed the payments he received from foreign sources after he was warned explicitly by the Pentagon," Cummings said. "Our next step is to get the documents we are seeking from the White House so we can complete our investigation."

But the White House told the committee recently that documents the lawmakers sought concerning Flynn's security clearance would not be turned over because they contained classified information.