Donald Trump Junior says he met with a Russian lawyer at the request of an acquaintance
The Kremlin has said it has no connection with the lawyer who met with Donald Trump Jr during the 2016 presidential campaign.
Natalia Veselnitskaya met with Donald Trump Jr and the president's son-in-law Jared Kushner at Trump Tower in June last year.
The meeting was first reported in The New York Times on Saturday.
Representatives of Mr Trump Jr and Jared Kushner, who is married to the businessman's daughter Ivanka, confirmed the meeting.
Mr Trump Jr said on Sunday that he had attended because he believed it would give him information helpful to his father's campaign.
Before that admission Mr Trump Jr had responded to the New York Times report by stating that the meeting was concerned with a US-Russia adoption law.
Paul Manafort, Mr Trump's campaign chief at the time, was also present at the meeting, the New York Times reported.
A spokesperson for Mr Trump's legal team, Mark Corallo, said President Trump was not aware of the meeting and did not attend.
Mr Trump Jr said he had invited Mr Kushner and Mr Manafort to attend the appointment but had not disclosed what it would involve.
In the longer statement, made on Sunday, he said he was told the meeting would be with an individual who "might have information helpful to the campaign".
He said that an acquaintance, known to him from the 2013 Miss Universe Pageant, had promised intelligence that would show "individuals connected to Russia were funding the Democratic National Committee".
During the meeting, he said, it became clear that the lawyer had "no meaningful information" and instead made vague statements before changing the subject to the adoption law.
He said he believed this had been the lawyer's reason for the meeting and that the promise of helpful information on the campaign was a pretext.
The New York Times describes Ms Veselnitskaya as known for her opposition to the Magnitsky Act, an 2009 law blacklisting Russian rights abusers.
The law provoked the anger of Russian President Putin and prompted him to stop the adoption of Russian children by US citizens.
The changing explanation comes as President Trump backtracked on suggestions that he would cooperate with Russia on a cybersecurity unit to prevent election meddling.
After tweeting on the subject on Sunday morning he attracted widespread ridicule over suspicions that Russia is suspected of having intervened in last year's Presidential race.
By the evening he had taken a different angle, tweeting "the fact that President Putin and I discussed a Cyber Security unit doesn't mean I think it can happen. It can't".