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The son of former national security adviser Michael Flynn is reportedly a subject of special counsel Robert Mueller's investigation into Russian interference in the 2016 presidential election. Four government officials told NBC News that the investigation is looking into Michael Flynn Jr.'s work for his father's lobbying firm, the Flynn Intel Group, which has been under scrutiny for its ties to Russia and Turkey.

Flynn Jr. was fired from Trump's transition team in December after the president reportedly became irritated by Flynn's persistent promotion of a false conspiracy theory alleging Hillary Clinton was behind a child sex-trafficking ring. The news of Flynn Jr.'s inclusion in the Russia probe comes one day after House Democrats sent a letter to Mueller raising concern about the former national security adviser's failure to disclose a 2015 trip to the Middle East, where Flynn senior may have lobbied on behalf of a foreign government to strike a deal to build a nuclear reactors program.

"The American people deserve to know whether General Flynn was secretly promoting the private interests of these businesses while he was a campaign adviser, a transition official, or President Trump's National Security Adviser," said the letter, signed by Rep. Elijah Cummings (D-Md.) and Rep. Eliot Engel (D-NY.).

In February, Flynn was fired as national security adviser after reports surfaced that he lied to Vice President Mike Pence about conversations he had had with the Russian ambassador, making him vulnerable to possible Russian blackmail.

Flynn Jr. remains active in attacking Clinton on social media. In recent days, he has retweeted several criticisms of Clinton and her new memoir, What Happened. He has yet to make a comment on being a subject of Mueller's federal inquiry.