Then-national security adviser Michael Flynn at the White House in February. (Carolyn Kaster/AP)

The lobbying activities of ­Michael G. Flynn, the son of President Trump's former national security adviser, are being examined by the special counsel investigating possible coordination between the Trump campaign and Russia during the 2016 election, according to people familiar with the probe.

The inquiry into the younger Flynn, first reported by NBC News, follows other indications this week that investigators are increasing pressure on his father, retired Army Lt. Gen. Michael ­Flynn, who advised Trump during the campaign and briefly served in the White House before being ousted for misleading statements about his contacts with the Russian ambassador.

Special counsel Robert S. Mueller III is looking at the younger Flynn because of his role as chief of staff to his father at the Flynn Intel Group, a lobbying and consulting firm that worked for international and domestic clients, according to the people familiar with the inquiry, who spoke on the condition of anonymity to discuss an open case.

Meanwhile, this week, two senior House Democrats requested information from the elder Flynn and some of his business partners about a joint U.S.-Russia proposal to sell nuclear power plants in the Middle East. The letter sent Tuesday by Reps. Elijah E. Cummings (D-Md.) and Eliot L. Engel (D-N.Y.) sought details of Flynn's travels to the Middle East in 2015 to promote the nuclear project — trips that the lawmakers said were not disclosed by Flynn in early 2016 as required.

"It appears that General Flynn violated federal law by omitting this trip and these foreign contacts from his renewal application" for a security clearance, the lawmakers wrote to Flynn's attorney and two business partners.

Flynn's attorney, Robert Kelner, declined to comment on the letter. An attorney for the younger Flynn, Barry Coburn, declined to comment on the NBC report.

Lawmakers have been inquiring in recent months about the elder Flynn's connections to Turkish, Saudi and Russian interests. The Flynns flew together to Moscow in 2015 when the elder Flynn delivered a paid speech at an anniversary celebration for RT television. While in Moscow, Flynn sat by Russian President Vladi­mir Putin at a dinner marking the network anniversary.

The lawmakers' interest in the nuclear power proposal was sparked by a June report by Newsweek detailing the plan to jointly market nuclear power plants to Israel, Egypt and other countries in the region. Newsweek reported that Flynn flew to Egypt and Israel in June 2015 "to gauge attitudes in Cairo and Jerusalem toward a plan for a joint U.S.-Russian (and Saudi financed) program to get control over the Arab world's rush to acquire nuclear power."

The letter requested details of any communication Flynn had with foreign governments about the nuclear project and suggested his interest in the proposed deals may have represented a conflict of interest. "The American people deserve to know whether General Flynn was secretly promoting the private interests of these businesses" while acting in his official capacity as adviser to the Trump campaign, the transition and the president, the letter said.

Although Flynn's attorney has rejected past efforts by Democratic lawmakers to gather information, they have persisted. In addition to requesting documents Tuesday, they asked that Flynn and his business partners appear before House committees for transcribed interviews with staffers. In the past, Flynn's attorney has pledged to cooperate with congressional intelligence committees, which have subpoena power and can compel testimony.