Top House Democrats asked the Defense Department on Wednesday to investigate whether President Donald Trump's national security adviser, Michael Flynn, violated the U.S. Constitution by accepting money from a Russian state television network to speak at a Moscow gala.

Flynn was seated next to Russian President Vladimir Putin at the 2015 event, which celebrated the 10th anniversary of state-sponsored news agency Russia Today.

“It is extremely concerning that General Flynn chose to accept payment for appearing at a gala hosted by the propaganda arm of the Russian government, which attacked the United States in an effort to undermine our election,” wrote Reps. Elijah Cummings, D-Md., Adam Smith, D-Wash., John Conyers, D-Mich., Bennie Thompson, D-Miss., Adam Schiff, D-Calif., and Eliot Engel, D-N.Y., in a letter to Secretary of Defense James Mattis.

“As a retired Army officer, General Flynn was prohibited from accepting direct or indirect payments from foreign governments,” they wrote, questioning whether Flynn violated the emoluments clause of the Constitution, which prohibits people holding office from accepting gifts from foreign governments.

The lawmakers asked the Pentagon to turn over any records on Flynn’s communications with the Russian government or Russia Today, as well as any recorded payments since he retired from the Army, by Feb. 16.

"It remains unclear how much Gen. Flynn was paid for his dinner with Vladimir Putin, whether he received additional payments from Russian or other foreign sources on separate occasions, or whether he sought the approval of the Department of Defense or Congress to accept any of these payments," they wrote Mattis.

The Pentagon declined to comment on the request for a probe on Wednesday, but will respond directly to the lawmakers, according to spokeswoman Laura Seal.

Flynn’s repeated contact with Russia’s ambassador to the U.S. was investigated by counterintelligence officials last year, after it was picked up by routine U.S. monitoring of the Russian diplomats. Since his retirement in 2014, Flynn has appeared several times on Russia Today.

Asked about the letter at Wednesday’s press briefing, White House press secretary Sean Spicer said it was a common practice for retired officers to give such speeches.

In an interview with the Washington Post last year, Flynn said he saw no conflict in being paid for speaking at the event, pointing out that it was arranged by his speaker’s bureau.

“I do public speaking. It was in Russia. It was a paid speaking opportunity," he said.