WASHINGTON — Investigations intensified into President Donald Trump’s ousted national security adviser, Michael Flynn, on Thursday as the Pentagon watchdog joined lawmakers in probing payments he accepted from foreign sources including a Russian state-sponsored TV network.

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, a former Army lieutenant general and Defense Intelligence Agency chief, later accepted tens of thousands of dollars for his work on behalf of foreign interests, including RT, the state-supported Russian television network, and a Turkish-owned company linked to Turkey’s government.

The Pentagon’s acting inspector general’s office confirmed Thursday he has launched an inquiry into whether those payments qualify as coming from foreign governments and whether Flynn properly informed military authorities about them.

The White House defended its hiring of Flynn and attempted to shift blame for any problems with his vetting onto the Obama administration, which handled the reissuance of his security clearance in January 2016.

Rep. Elijah Cummings, D-Md., who released the documents, said during a news conference that Flynn had been clearly informed he needed to get permission to receive foreign payments and there’s no evidence he did so.

“The Pentagon’s warning to General Flynn was bold, italicized and could not have been clearer,” Cummings said.

In a key 2014 document, Flynn was told by a Defense Intelligence Agency official 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 Oct. 8, 2014, letter — which was sent to Flynn at his request — explained that such “advance approval” would need to come “from the relevant service secretary.”

Earlier this week, Cummings and Republican Rep. Jason Chaffetz, chairman of the House committee, said that they had found no evidence Flynn asked the Army for permission to receive foreign payments or informed the military he had accepted them. Army spokeswoman Cynthia O. Smith said the Army had no records that Flynn requested that permission.

One episode in question involves a trip he took to Moscow in 2015 for RT’s anniversary celebration. He was paid at least $33,750 to attend the gala at which he was seated next to Russian President Vladimir Putin.