The son of former national security adviser Mike Flynn is now the subject of the Justice Department's investigation into Russia's interference in the 2016 election, according to a report.

Special counsel Robert Mueller and his team of prosecutors are looking into Michael G. Flynn and, in part, his work with the elder Flynn's lobbying firm, called the Flynn Intel Group, sources told NBC News.

Mike Flynn, former director of the Defense Intelligence Agency and a retired Army lieutenant general, is already under investigation by both the federal government and Congress for his ties to foreign governments, including Russia and Turkey.

President Trump fired the elder Flynn from his job as national security adviser in February after he lied to Vice President Mike Pence about his contacts with then-Russian Ambassador to the U.S. Sergei Kislyak.

Sources told NBC News Michael G. Flynn worked closely with the elder Flynn at Flynn Intel Group and served as his chief of staff. He participated in meetings with Mike Flynn and talked with potential clients, a former business associate of the elder Flynn told NBC News.

The duo traveled to Moscow together in December 2015, where Mike Flynn gave a speech at a gala for RT, the state-sponsored television network.

The younger Flynn was spotted in a video from the event, and Mike Flynn sat at a table with Russian President Vladimir Putin.

The elder Flynn was paid close to $34,000 for the speech, according to documents from the House Oversight and Government Reform Committee, and RT also covered the father and son's airfare to Moscow, as well as their lodging from Dec. 9 to Dec. 12.

The elder Flynn and his company are under scrutiny for ties to foreign governments, including work Mike Flynn was paid $530,000 for last year that benefited the Turkish government.

Mike Flynn wasn't registered as a foreign lobbyist at the time, but retroactively filed with the Justice Department earlier this year.