WASHINGTON — Michael G. Flynn, the son of President Donald Trump's former national security adviser, is a subject of the federal investigation into Russian meddling in the presidential election and possible collusion between Moscow and the Trump campaign, according to four current and former government officials.

The inquiry into Flynn is focused at least in part on his work with his father's lobbying firm, Flynn Intel Group, three of the officials said. It's unclear when the focus on Flynn began.

Barry Coburn, who said he is serving as the younger Flynn's legal counsel, said he couldn't comment on the matter.

Flynn's status as a subject of the Russia investigation widens the publicly known scope of the probe. NBC News has reported that those under investigation have included the elder Flynn and former Trump campaign manager Paul Manafort. Others under scrutiny by special counsel Robert Mueller include Carter Page, a Trump campaign ally; Jared Kushner, the president's son-in-law and senior White House adviser; and the president's son, Donald Trump Jr.

Trump Jr. may be called to appear before the Senate Judiciary committee sometime this fall. He, Kushner, Manafort and Page have all denied any collusion with Russia during the campaign.

Ty Cobb, an attorney for President Trump, said that the younger Flynn's status as a subject of the probe "does not impact the White House to any extent with regard to its continuing cooperation with the special counsel."

The elder Flynn's lawyer, Robert Kelner, declined to comment Tuesday when asked how his client responds to allegations of collusion with Russia. On Twitter, Michael G. Flynn appeared to call the report that he is a subject of the Russia probe a "#Nothingburger": "#FakeNews Media: 'We're done covering those "pesky hurricanes' right????...Back to Russia!' #Nothingburger."

The president also has come under scrutiny for possible obstruction of justice and has denied colluding with Russia.

The younger Flynn worked closely with his father, whose connections to foreign governments, including Russia and Turkey, have been a subject of federal and congressional investigations.

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Michael G. Flynn accompanied his father, for instance, on a trip to Moscow in December 2015 for the elder Flynn to deliver a paid speech at a 10th anniversary celebration for the state-sponsored Russian television network RT. The younger Flynn can be seen in video from an associated event.

The elder Flynn, a retired Army lieutenant general and a former director of the Defense Intelligence Agency, was seated at the same table as Russian President Vladimir Putin during the dinner. Records released by the House Oversight Committee show he was paid nearly $34,000 for his speech; RT also paid for Flynn and his son's airfare to Moscow and lodging at a luxury hotel from Dec. 9 to Dec. 12.

Flynn Intel Group also was paid $530,000 in 2016 for work the Justice Department has said benefited the government of Turkey. The elder Flynn did not register as a foreign lobbyist at the time, but did so retroactively this year. According to his filing with the Justice Department, he was hired by a Turkish businessman to gather information about Fethullah Gülen, a Turkish cleric residing in Pennsylvania whom the Turkish government accuses of orchestrating an attempted coup there in July 2016.

Related: Obama Warned Trump Against Hiring Mike Flynn

The elder Flynn was fired as Trump's national security adviser in February after it became public that he had misled Vice President Mike Pence about his conversations with the Russian ambassador to the U.S.

A former business associate of Michael Flynn's said the younger Flynn had a heavy hand in the day-to-day operations of Flynn Intel Group and served as his father's chief of staff. Those responsibilities included attending meetings with his father and communicating with prospective clients, the former business associate said.

Several legal experts with knowledge of the investigation have told NBC News they believe Mueller, following a classic prosecutorial playbook, is seeking to compel key players, including Flynn and Manafort, to tell what they know about any possible Trump campaign collusion with Russia. Mueller has brought onto his team a federal prosecutor known for convincing subjects to turn on associates. Any potential criminal liability for Michael G. Flynn could put added pressure on his father, these legal experts said.

"Any time a family member is identified as a subject that does increase pressure," said Peter White, a former federal prosecutor. "In the typical parent-child relationship the last thing any parent would want is for their child to get in trouble for something they initiated."

That pressure appeared to mount Wednesday, when House Democrats released information they said confirmed that the elder Flynn omitted from his security clearance renewal application in 2016 that he had traveled to the Middle East in 2015 to meet with foreign leaders about a proposal to partner with Russia in a plan to build nuclear reactors in Saudi Arabia.

In a letter to Flynn's former business partners who had turned over documents to Congress, Democratic Reps. Elijah Cummings of Maryland and Eliot Engel of New York also accuse Flynn of concealing the trip from background check investigators who interviewed him during that 2016 process. The congressmen, ranking members on the House Oversight and House Foreign Affairs committees respectively, told the former business partners that because "it appears that General Flynn violated federal law" they are turning over their documents to Mueller.

Related: Flynn, Manafort Are Key Figures in Russia Probe

Peter Carr, the spokesman for Mueller, declined to comment. Kelner, the elder Flynn's lawyer, did not respond to a request for comment on the Cummings-Engel letter.

The younger Flynn, 34, has a bachelor's degree from the University of North Carolina-Charlotte and an associate degree in golf course management. He is married, has one son and lives in Northern Virginia. He worked for a golf company and then a healthcare management firm between 2008 and 2015, and since 2014 has worked for the Flynn Intel Group, according to LinkedIn. He was a registered Republican in Charlotte, N.C., from 2004 to 2008, according to public records.

He was a controversial figure during the presidential campaign and during the Trump transition, known for writing inflammatory comments on Twitter and circulating conspiracy theories.

He perpetuated a so-called "pizzagate" conspiracy theory that surfaced in the days before the November election alleging Democratic candidate Hillary Clinton had used the Comet Ping Pong pizzeria in Washington to run a child sex-trafficking operation. About a month later, in December 2016, a North Carolina man fired gunshots in the pizzeria allegedly in response to the false "pizzagate" story. At the time, Flynn wrote on Twitter: "Until #Pizzagate proven to be false, it'll remain a story."

During the Trump transition there were questions about whether Flynn had an official role. He had a government transition email address and was said to be helping his father, who had been named national security adviser. But after the shooting at the "pizzagate" restaurant, Pence, who was in charge of the transition, said Flynn "has no involvement in the transition whatsoever."

The younger Flynn continues to express political opinions on social media, tweeting in support of former Trump adviser Steve Bannon and tweeting criticism of Black Lives Matter.