The Department of Defense has begun investigating whether President Donald Trump's former National Security Adviser Michael Flynn accepted money from foreign governments without obtaining the necessary approval, according to a letter made public on Thursday by the top Congressional democrat on the House Committee on Oversight and Government Reform.

The letter, one of three documents released by Maryland Democrat Elijah Cummings, revealed that Defense Department Inspector General Glenn Fine is examining whether Flynn "failed to obtain required approval prior to receiving any emolument from a foreign government." Fine sent the letter on April 11 to the Committee Chairman and Republican Representative Jason Chaffetz of Utah.

Three documents, one infringement

The other two documents released by Cummings also pertain to foreign payments received by Flynn.

An October 2014 letter from the Defense Department to Flynn upon his military retirement warned that the former US Army lieutenant general and Defense Intelligence Agency (DIA) director would need "advance approval" coming "from the relevant service secretary" before being cleared to receive payments linked to foreign governments.

This is because the US Constitution's Title of Nobility Clause states "no person holding any office of profit or trust under them, shall, without the consent of the Congress, accept of any present, emolument, office, or title, of any kind whatever, from any king, prince, or foreign state" - a clause intended to protect against foreign influence in US government.

The final letter that Cummings released, an unclassified version of a DIA letter sent to the committee on April 7, indicated that Flynn had failed to follow proper procedures as Pentagon officials had no record of him seeking permission for foreign funds he received.

"DIA did not locate any records referring or relating to LTG (Lieutenant General) Flynn's receipt of money from a foreign source...DIA did not locate any records of LTG Flynn seeking permission or approval for the receipt of money from a foreign source,” the redacted letter said.

Money from Russia and Turkey

At the heart of the Pentagon's investigation are payments Flynn received from Russian and Turkish interests. In 2015, Flynn received at least $33,750 (31,035 euros) to appear at a gala dinner for the Kremlin-controlled Russian media outlet RT. That fall, he was also paid over $500,000 for work undertaken as a foreign agent for a Turkish firm with links to the Turkish government.

In releasing the letters on Thursday, Cummings said there was "no evidence, not a shred, that [Flynn] disclosed his payments from RT, the Russian propaganda outlet."

However, Flynn's lawyer maintained the former defense official briefed the DIA before traveling to Russia, a claim which contradicts the letter the DIA sent to Congress.

Both Cummings and Chaffetz, who was not present at the documents' release on Thursday, said earlier this week that Flynn likely broke the law.

Read more: US lawmakers push for information on Flynn's Russia ties

An addition to Trump-Russian intrigue

Flynn stepped down as Trump's national security adviser on February 13 after lying about contact he had with Russian officials. His resignation came amidst investigations by the FBI and Congress into possible relations between Russian officials and the Trump campaign during the 2016 presidential election and transition period.

In his remarks on Thursday, Cummings called directly on the White House to release documents pertaining to Flynn. The White House has refused requests to turn these over, citing classified information.

However, White House spokesman Sean Spicer reacted to the news of the Pentagon's investigation positively. 

"That's appropriate. If they think that there's wrongdoing, the department's inspector general should look into it. We welcome that," Spicer said.

cmb/sms (Reuters, AP, dpa)