Russian President Vladimir Putin with Michael Flynn, center left, during an exhibition in Moscow celebrating the RT television network on Dec. 10, 2015. (Poll photo by Mikhail Klimentyev/Sputnik via AP)

The ranking Democrats on six House congressional committees asked the Pentagon on Wednesday for information about President Trump’s national security adviser, suggesting that he may have violated a constitutional restriction by accepting a fee for speaking at a 2015 Moscow event.

Michael Flynn, a retired Army lieutenant general, sat with Russian President Vladi­mir Putin during the event, a celebration for the Kremlin-controlled RT television network.

The lawmakers suggest that the fee he received may have violated the Constitution’s emoluments clause, which prohibits top officials from receiving payments from foreign governments.

National security adviser Michael Flynn, center, listens to President Trump during a session with cybersecurity experts in the Roosevelt Room of the White House on Jan. 31. (Jabin Botsford/The Washington Post)

“It is extremely concerning that [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,” the members wrote in the letter sent Wednesday.

The letter, signed by the ranking Democrats on the House Intelligence, Oversight, Armed Services, Judiciary, Foreign Affairs and Homeland Security committees, requested any information the Pentagon had about “how much [Flynn] was paid for his dinner with Vladi­mir Putin, whether he received additional payments from Russian or other foreign sources, or whether he sought the approval of the Department of Defense or Congress to accept these payments.”

White House press secretary Sean Spicer said during a news briefing Wednesday that “Flynn, like countless, if not hundreds, of retired flag officers joined a speakers bureau and have given speeches at various places. I think that is something that is kept in practice.” Defense Department officials did not respond to requests for comment on the congressional letter.

A former White House ethics counsel, Norman Eisen, said the questions are “troubling and important.”

“Concern about the alleged ties between the Trump administration and Russia, and its effect on our national security, is precisely the kind of worry that led the founders to include this prohibition in the Constitution,” said Eisen, who served as ethics adviser to President Barack Obama and who has emerged as a sharp critic of Trump administration ethics policy. He filed a lawsuit with others accusing Trump of violating the emoluments clause.

In the past, the Pentagon has advised retiring officers that because they can be recalled to military service, they may be subject to the Constitution’s emoluments clause.

“If Lieutenant General Flynn is subject to recall [by the Defense Department] and accepted foreign government funds without congressional consent, then that is a foreign emolument and so a violation of the Constitution,” Eisen said, adding, “Nor would it be a merely technical one.”

The Defense Department has warned retired officers against accepting “indirect payments, including from pass-through companies in the United States,” in retirement, the congressional letter said.

Flynn has been among the more controversial of Trump’s appointees not only for his trip to Russia but also because he has made inflammatory statements in previously published tweets. In November, for example, he tweeted, “Fear of Muslims is RATIONAL: please forward this to others: the truth fears no questions.”

He was removed from his command of the Defense Intelligence Agency in 2014 because of concerns over his leadership style. While he was working for the Trump campaign, his firm was also paid as a lobbyist for a consultancy founded by a Turkish businessman.

Whether he received permission to receive the Russia payment from Congress or the Defense Department may be important, the representatives wrote, because the Constitution prohibits any person “holding any Office of Profit or Trust” from accepting gifts or payments from any foreign country. The Defense Department has made clear this restriction applies to retired military officers, who continue to hold offices of trust.

The letter, signed by Reps. Adam B. Schiff (D-Calif.), Elijah E. Cummings (D-Md.) and others, quotes Defense Department instructions to retired officers that says that permission is required. “Significantly, retired regular military officers are also subject to the Emoluments Clause because they are subject to recall, and, therefore, hold an ‘Office of Profit or Trust’ under the Emoluments Clause,” the letter notes.

Greg Miller and Alice Crites contributed to this report.