Donald Trump Jr confirmed reports that he was seeking compromising information on Clinton when he met Russian attorney Natalia Veselnitskaya in June 2016.
"Obviously, I'm the first person on a campaign to ever take a meeting to hear info about an opponent," the president's son said sardonically in a tweet.
He claimed that the meeting "went nowhere" but he "had to listen."
The FBI and Congress are investigating whether the Trump campaign colluded with Russia to influence the 2016 presidential vote.
US intelligence agencies have concluded that Russian President Vladimir Putin approved a mass effort to tilt the election in Trump's favor, including hacking and leaking embarrassing emails from Democrats.
The New York Times reported that Paul Manafort and Jared Kushner were also at the June 2016 meeting.
The pair were two of Trump's most trusted campaign officials, and Kushner -- also the president's son-in-law -- became part of the White House inner circle after the inauguration.
In a potentially even more damaging revelation, the New York Times reported later Monday that the younger Trump had been informed via email ahead of the meeting that material from the Russian lawyer was part of an effort by Moscow to aid his father's presidential bid and hurt Clinton.
Trump Jr's attorney Alan Futerfas called the reports "much ado about nothing."
In a statement provided to AFP, Futerfas said the younger Trump had been contacted by an individual named Rob Goldstone, described by the Times as a publicist and former British tabloid reporter.
Goldstone suggested in an email that "people had information concerning alleged wrongdoing by Democratic Party front runner, Hillary Clinton, in her dealings with Russia," Futerfas said.
Futerfas said Trump Jr. did not know what specific information would be discussed during the meeting, which lasted 20 to 30 minutes.
"Nothing came of it. His father knew nothing about it. The bottom line is that Don Jr did nothing wrong," Futerfas said, adding that his client would cooperate with any investigation.
Veselnitskaya is not well-known in Russia, but represented the son of a senior Russian official involved in a major fraud probe that had international repercussions.
In a statement to the Times, Trump Jr said she "stated that she had information that individuals connected to Russia were funding the Democratic National Committee and supporting Mrs Clinton."
The meeting took place at a pivotal point in the 2016 campaign, after Trump had shocked Republicans with a string of primary victories and Clinton had all-but secured the Democratic nomination.
"The president was not aware of and did not attend the meeting," Mark Corallo, a spokesman for President Trump's private lawyer, told AFP.
White House spokeswoman Sarah Huckabee Sanders said Trump had only learned of the meeting "in the last couple of days."
Trump Jr's meeting has already drawn the attention of Congressional investigators.
"Our intelligence committee needs to interview him and others who attended the meeting," said Republican Senator Susan Collins.
The Kremlin said it "did not know" who Veselnitskaya was. "We can't be aware of all the meetings Russian lawyers have abroad," said spokesman Dmitry Peskov.
Trump and Putin
President Trump has repeatedly decried allegations of collusion as "fake news" and equivocated about whether Russia tried to interfere in the election.
During a meeting with Putin on Friday on the margins of the G20 summit in Germany, Trump raised the issue of election interference before turning to other subjects.
"I strongly pressed President Putin twice about Russian meddling in our election," Trump later said. "He vehemently denied it. I've already given my opinion..."
That meeting in Hamburg is raising yet more Russia-related problems for the White House.
Scrambling for concrete victories to take away from the two-hour-plus sit-down, the White House hailed deals with Russia for establishing a joint "cyber unit" and a ceasefire in southern Syria.
The cyber project came under heavy political fire in Washington in the light of Russia's alleged hacking in the election campaign. It appeared to quickly unravel when Trump himself said he was unsure whether it would come into effect.
"The fact that President Putin and I discussed a Cyber Security unit doesn't mean I think it can happen. It can't -- but a ceasefire can, & did!" he tweeted.
(Except for the headline, this story has not been edited by NDTV staff and is published from a syndicated feed.)
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