But the latest revelation in the burgeoning scandal has added to the pressure on another family member who was at the meeting and is already in the crosshairs of investigators -- Jared Kushner, the president's influential son-in-law.
Democrats are up in arms, demanding that the 36-year-old Kushner -- a senior adviser to the president with an office in the White House -- be stripped of his security clearance.
"There doesn't seem to be any ethical standard in the White House," Democratic House minority leader Nancy Pelosi tweeted on Thursday.
"Jared Kushner's security clearance must be immediately revoked."
Even some from Donald Trump's Republican Party are not so sure that Kushner -- who is married to the president's eldest daughter Ivanka -- should remain in the West Wing.
"I'm going out on a limb here - but I would say I think it would be in the president's best interest if he removed all of his children from the White House," Texas Representative Bill Flores told local television Thursday.
"Not only Donald Trump (Jr.), but Ivanka and Jared Kushner."
Omissions on security clearance forms
While Donald Jr has no role in his father's administration -- he is helping run his corporate empire -- Kushner is one of Donald Trump's closest advisors.
The Harvard graduate is also the progeny of a powerful New York real estate family, and has long been in Donald Trump's inner circle.
It was actually an omission on a government security clearance application filed by Jared Kushner that led to the revelation of the meeting between himself, Donald Jr, former Trump campaign manager Paul Manafort and the Russian lawyer.
It also led Donald Jr to release an email chain about the planning of that meeting -- which is now being cited as the most serious evidence yet of alleged collusion between the Trump presidential campaign and Russia.
In the June 2016 emails, Donald Jr eagerly agrees to a meeting with a Kremlin-linked Russian lawyer who is said to possess incriminating information about Clinton and invites Kushner and Manafort to come along.
Jared Kushner, filing a security clearance document known as an SF-86, initially neglected to mention that he attended the meeting with lawyer Natalia Veselnitskaya -- as well as contacts he had with several other Russians, including Moscow's ambassador to the United States, Sergey Kislyak.
The meeting with Veselnitskaya came to light only after Kushner filed an amended SF-86 form.
Russia ties under microscope
But even before revelations of the Veselnitskaya meeting came to light, Kushner's other dealings with Russian officials have been facing scrutiny.
According to The Washington Post, Kushner -- at a December 2016 meeting with Kislyak -- raised the possibility of setting up a secret and secure communications link between the Trump transition team and the Kremlin.
That same month, Kushner also met with Sergey Gorkov, chief of Vnesheconombank and a former member of Russian intelligence. The bank, a key arm of the Russian government, is under tough US sanctions.
The Post reported last month that Kushner's finances and business dealings were being examined as part of the probe led by special counsel and former FBI director Robert Mueller into whether the Trump campaign teamed with Russia to help tilt the presidential race in favor of the billionaire tycoon.
And this week, the McClatchy newspaper group reported that congressional and Justice Department investigators were looking into whether the Trump campaign helped Russian cyber operatives bombard key voting districts with "fake news" about Clinton.
Kushner was in charge of the Trump campaign's digital operations. He now plays a major role in shaping foreign policy.
US intelligence agencies have concluded that Russian President Vladimir Putin approved a massive effort to swing the election to Donald Trump, including hacking and leaking embarrassing emails from Democrats.
President Donald Trump has vehemently denied any collusion between his campaign and Russia and repeatedly claimed to be the victim of a "witch hunt" by the media and sore loser Democrats.
Kushner is expected to discuss his Russian contacts at some point with the Senate Intelligence Committee, which is leading one of the several probes into Russian election interference.
But ahead of that testimony -- and with unanswered questions mounting around him -- opposition Democrats are demanding action now.
"It is unclear why Mr. Kushner continues to have access to classified information while these allegations are being investigated," said a letter from nearly 20 members of the House Oversight Committee sent to the White House last month.
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