And while President Trump has been dogged by revelations of undisclosed meetings between his associates and the Russians, the episode at Trump Tower is the first such confirmed private meeting involving members of his inner circle during the campaign — as well as the first one known to have included his eldest son. It came at an inflection point in the campaign, when Donald Trump Jr., who served as an adviser and a surrogate, was ascendant and Mr. Manafort was consolidating power.
It is unclear whether the Russian lawyer, Natalia Veselnitskaya, actually produced the promised compromising information about Mrs. Clinton. But the people interviewed by The Times about the meeting said the expectation was that she would do so.
In a statement on Sunday, Donald Trump Jr. said he had met with the Russian lawyer at the request of an acquaintance. “After pleasantries were exchanged,” he said, “the woman stated that she had information that individuals connected to Russia were funding the Democratic National Committee and supporting Ms. Clinton. Her statements were vague, ambiguous and made no sense. No details or supporting information was provided or even offered. It quickly became clear that she had no meaningful information.”
He said she then turned the conversation to adoption of Russian children and the Magnitsky Act, an American law that blacklists suspected Russian human rights abusers. The law so enraged President Vladimir V. Putin of Russia that he retaliated by halting American adoptions of Russian children.
“It became clear to me that this was the true agenda all along and that the claims of potentially helpful information were a pretext for the meeting,” Mr. Trump said.
When he was first asked about the meeting on Saturday, he said only that it was primarily about adoptions and mentioned nothing about Mrs. Clinton.
Mark Corallo, a spokesman for the president’s lawyer, said on Sunday that “the president was not aware of and did not attend the meeting.”
Lawyers and spokesmen for Mr. Kushner and Mr. Manafort did not immediately respond to requests for comment. In his statement, Donald Trump Jr. said he asked Mr. Manafort and Mr. Kushner to attend, but did not tell them what the meeting was about.
American intelligence agencies have concluded that Russian hackers and propagandists worked to tip the election toward Donald J. Trump, in part by stealing and then providing to WikiLeaks internal Democratic Party and Clinton campaign emails that were embarrassing to Mrs. Clinton. WikiLeaks began releasing the material on July 22.
A special prosecutor and congressional committees are now investigating the Trump campaign’s possible collusion with the Russians. Mr. Trump has disputed that, but the investigation has cast a shadow over his administration.
Mr. Trump has also equivocated on whether the Russians were solely responsible for the hacking. On Sunday, two days after his first meeting as president with Mr. Putin, Mr. Trump said in a Twitter post: “I strongly pressed President Putin twice about Russian meddling in our election. He vehemently denied it. I’ve already given my opinion…..” He also tweeted that they had “discussed forming an impenetrable Cyber Security unit so that election hacking, & many other negative things, will be guarded…””
On Sunday morning on Fox News, the White House chief of staff, Reince Priebus, described the Trump Tower meeting as a “big nothing burger.”
“Talking about issues of foreign policy, issues related to our place in the world, issues important to the American people is not unusual,” he said.
But Representative Adam B. Schiff of California, the leading Democrat on the House Intelligence Committee, one of the panels investigating Russian election interference, said he wanted to question “everyone that was at that meeting.”
“There’s no reason for this Russian government advocate to be meeting with Paul Manafort or with Mr. Kushner or the president’s son if it wasn’t about the campaign and Russia policy,” Mr. Schiff said after the initial Times report.
Ms. Veselnitskaya, the Russian lawyer invited to the Trump Tower meeting, is best known for mounting a multipronged attack against the Magnitsky Act.
The adoption impasse is a frequently used talking point for opponents of the act. Ms. Veselnitskaya’s campaign against the law has also included attempts to discredit the man after whom it was named, Sergei L. Magnitsky, a lawyer and auditor who died in 2009 in mysterious circumstances in a Russian prison after exposing one of the biggest corruption scandals during Mr. Putin’s rule.
Ms. Veselnitskaya’s clients include state-owned businesses and a senior government official’s son, whose company was under investigation in the United States at the time of the meeting. Her activities and associations had previously drawn the attention of the F.B.I., according to a former senior law enforcement official.
Ms. Veselnitskaya said in a statement on Saturday that “nothing at all about the presidential campaign” was discussed. She recalled that after about 10 minutes, either Mr. Kushner or Mr. Manafort walked out.
She said she had “never acted on behalf of the Russian government” and “never discussed any of these matters with any representative of the Russian government.”
The Trump Tower meeting was disclosed to government officials in recent days, when Mr. Kushner, who is also a senior White House aide, filed a revised version of a form required to obtain a security clearance.
The Times reported in April that he had failed to disclose any foreign contacts, including meetings with the Russian ambassador to the United States and the head of a Russian state bank. Failure to report such contacts can result in a loss of access to classified information and even, if information is knowingly falsified or concealed, in imprisonment.
Mr. Kushner’s advisers said at the time that the omissions were an error, and that he had immediately notified the F.B.I. that he would be revising the filing.
In a statement on Saturday, Mr. Kushner’s lawyer, Jamie Gorelick, said: “He has since submitted this information, including that during the campaign and transition, he had over 100 calls or meetings with representatives of more than 20 countries, most of which were during transition. Mr. Kushner has submitted additional updates and included, out of an abundance of caution, this meeting with a Russian person, which he briefly attended at the request of his brother-in-law Donald Trump Jr. As Mr. Kushner has consistently stated, he is eager to cooperate and share what he knows.”
Mr. Manafort, the former campaign chairman, also recently disclosed the meeting, and Donald Trump Jr.’s role in organizing it, to congressional investigators who had questions about his foreign contacts, according to people familiar with the events. Neither Mr. Manafort nor Mr. Kushner was required to disclose the content of the meeting.
A spokesman for Mr. Manafort declined to comment.
Since the president took office, Donald Trump Jr. and his brother Eric have assumed day-to-day control of their father’s real estate empire. Because he does not serve in the administration and does not have a security clearance, Donald Trump Jr. was not required to disclose his foreign contacts. Federal and congressional investigators have not publicly asked for any records that would require his disclosure of Russian contacts.
Ms. Veselnitskaya is a formidable operator with a history of pushing the Kremlin’s agenda. Most notable is her campaign against the Magnitsky Act, which provoked a Cold War-style, tit-for-tat dispute with the Kremlin when President Barack Obama signed it into law in 2012.
Under the law, about 44 Russian citizens have been put on a list that allows the United States to seize their American assets and deny them visas. The United States asserts that many of them are connected to the fraud exposed by Mr. Magnitsky, who after being jailed for more than a year was found dead in his cell. A Russian human rights panel found that he had been assaulted. To critics of Mr. Putin, Mr. Magnitsky, in death, became a symbol of corruption and brutality in the Russian state.
An infuriated Mr. Putin has called the law an “outrageous act,” and, in addition to banning American adoptions, he compiled what became known as an “anti-Magnitsky” blacklist of United States citizens.
Among those blacklisted was Preet Bharara, then the United States attorney in Manhattan, who led notable convictions of Russian arms and drug dealers. Mr. Bharara was abruptly fired in March, after previously being asked to stay on by President Trump.
One of Ms. Veselnitskaya’s clients is Denis Katsyv, the Russian owner of Prevezon Holdings, an investment company based in Cyprus. He is the son of Petr Katsyv, the vice president of the state-owned Russian Railways and a former deputy governor of the Moscow region. In a civil forfeiture case prosecuted by Mr. Bharara’s office, the Justice Department alleged that Prevezon had helped launder money linked to the $230 million corruption scheme exposed by Mr. Magnitsky by putting it in New York real estate and bank accounts. Prevezon recently settled the case for $6 million without admitting wrongdoing.
Ms. Veselnitskaya and her client also hired a team of political and legal operatives to press the case for repeal. And they tried but failed to keep Mr. Magnitsky’s name off a new law that takes aim at human-rights abusers across the globe. The team included Rinat Akhmetshin, an émigré to the United States who once served as a Soviet military officer and who has been called a Russian political gun for hire. Fusion GPS, a consulting firm that produced an intelligence dossier that contained unverified allegations about Mr. Trump, was also hired to do research for Prevezon.
Ms. Veselnitskaya was also deeply involved in the making of a film that disputes the widely accepted version of Mr. Magnitsky’s life and death. In the film and in her statement, she said the true culprit of the fraud was William F. Browder, an American-born financier who hired Mr. Magnitsky to investigate the fraud after three of his investment funds companies in Russia were seized.
Mr. Browder called the film a state-sponsored smear campaign.
“She’s not just some private lawyer,” Mr. Browder said of Ms. Veselnitskaya. “She is a tool of the Russian government.”
John O. Brennan, a former C.I.A. director, testified in May that he had been concerned last year by Russian government efforts to contact and manipulate members of Mr. Trump’s campaign. “Russian intelligence agencies do not hesitate at all to use private companies and Russian persons who are unaffiliated with the Russian government to support their objectives,” he said.
The F.B.I. began a counterintelligence investigation last year into Russian contacts with any Trump associates. Agents focused on Mr. Manafort and a pair of advisers, Carter Page and Roger J. Stone Jr.
Among those now under investigation is Michael T. Flynn, who was forced to resign as Mr. Trump’s national security adviser after it became known that he had falsely denied speaking to the Russian ambassador about sanctions imposed by the Obama administration over the election hacking.
Congress later discovered that Mr. Flynn had been paid more than $65,000 by companies linked to Russia, and that he had failed to disclose those payments when he renewed his security clearance and underwent an additional background check to join the White House staff.
In May, the president fired the F.B.I. director, James B. Comey, who days later provided information about a meeting with Mr. Trump at the White House. According to Mr. Comey, the president asked him to end the bureau’s investigation into Mr. Flynn; Mr. Trump has repeatedly denied making such a request. Robert S. Mueller III, a former F.B.I. director, was then appointed as special counsel.
The status of Mr. Mueller’s investigation is not clear, but he has assembled a veteran team of prosecutors and agents to dig into any possible collusion.