It was in that capacity, Mr. Akhmetshin said in an interview on Friday, that he accompanied a Russian lawyer, Natalia Veselnitskaya, to the meeting as part of an effort to amend an American law known as the Magnitsky Act that sanctioned Russians for human rights abuses. The 2012 law infuriated President Vladimir V. Putin of Russia, whose government retaliated by restricting adoptions of Russian children by Americans.
Mr. Akhmetshin, who wears his curly hair high in a manner that associates compare to the protagonist in the 1977 cult-classic horror film “Eraserhead,” said in the interview that he took part in the meeting at the request of Ms. Veselnitskaya. He said he had no ties to the Kremlin.
“I am a target of well-coordinated and financed smear campaign,’’ he said in a text message.
He described his time in the military as routine, serving from 1986 to 1988, like “millions of other Soviet boys.” He said he left the military with the rank of sergeant.
NBC News first reported Mr. Akhmetshin’s role in the Donald Trump Jr. meeting, but did not identify him. He first confirmed to The Associated Press that he attended the meeting.
Mr. Akhmetshin has boasted to associates that he had served in the military with a group known as the Osoby Otdel, or Special Section, which in the Soviet period was a division of the K.G.B. The group was distinct from the G.R.U., or Main Intelligence Directorate of the defense ministry, an organization with which he has denied any affiliation.
Those who know Mr. Akhmetshin describe him as a devoted parent, ferrying his school-age daughter to extracurricular activities around Washington. He speaks with a slight accent and is a voracious reader of classic American novels.
Mr. Akhmetshin emigrated to the United States in 1994 to attend graduate school, he said. He became involved in Washington advocacy work in 1998 when he responded to an ad, leading to a job running an organization pushing what he described as a pro-democracy agenda for Kazakhstan. The position was arranged by Edward Lieberman, a Washington lawyer who has extensive business experience in Russia.
The veteran investigative journalist Seymour Hersh, who met Mr. Akhmetshin through Mr. Lieberman, said that despite Mr. Akhmetshin’s work to repeal the Magnitsky Act, he is cleareyed about the Russian government.
“He is not a whore for Putin,” said Mr. Hersh, who moderated a panel arranged by Mr. Akhmetshin after a Washington screening of a film critical of the dead Russian accountant for whom the Magnitsky Act is named. Mr. Hersh said Mr. Akhmetshin cared a great deal about getting the restrictions on adoptions rescinded.
Since The New York Times first reported the meeting on July 8, Donald Trump Jr. has had shifting accounts of what happened. When he learned that The Times planned to publish excerpts from his emails about the meeting, Donald Trump Jr. released them himself on Twitter and said he was doing so to be transparent.
The emails revealed that Donald Trump Jr. also invited Jared Kushner, the president’s son-in-law and a campaign adviser, as well as Paul J. Manafort, who at the time was the Trump campaign chairman. Rob Goldstone, a publicist and former British tabloid reporter who helped broker the meeting, was also there.
But Donald Trump Jr. did not disclose that other people who were not mentioned in the emails were also in attendance, including Mr. Akhmetshin and a translator. In his interview with The Times, Mr. Akhmetshin identified the translator as Anatoli Samachornov.
Mr. Samachornov, in an interview with The Times on Monday, declined to say whether he had been in the meeting. But he did say that he was working with Ms. Veselnitskaya at the time of the Trump Tower meeting on efforts related to the Magnitsky Act and adoptions.
Alan S. Futerfas, Donald Trump Jr.’s attorney, suggested in an interview on Friday that his client has not disclosed all the details of the meeting because it was hard to remember. “The frustrating part of this exercise is that it concerns events that occurred 13 months ago that were considered insignificant at the time and essentially forgotten,” Mr. Futerfas said.
Mr. Akhmetshin said he lobbies for a nonprofit group, the Human Rights Accountability Global Initiative Foundation, that records show is affiliated with Russian businessmen who have ties to the Russian government, including Denis Katsyv, whose businesses have been represented by Ms. Veselnitskaya.
The foundation registered to lobby the United States Congress on issues related to the Magnitsky Act and adoptions two days after the Trump Tower meeting.
Donald Trump Jr.’s team initially said the June 2016 meeting focused on those issues. Yet Ms. Veselnitskaya and Mr. Akhmetshin did not mention the meeting to most members of their lobbying team working on the issue, according to the lobbyists.
A spokesman for Cozen O’Connor Public Strategies, one of the lobbying firms, said they were unaware of the June 2016 meeting — or the participation of Ms. Veselnitskaya and Mr. Akhmetshin — until media reports revealed it.
The Justice Department contacted Mr. Akhmetshin in March and asked him why he did not register his work for the nonprofit group under the Foreign Agent Registration Act, which requires anyone who lobbies in the United States on behalf of foreign interests to disclose their work to the Justice Department. Mr. Akhmetshin responded to the Justice Department in April, saying he had properly registered under congressional lobbying rules.
In 2015, International Mineral Resources, a mining company based in the Netherlands, accused Mr. Akhmetshin of hacking into its computer systems, stealing confidential information and unlawfully disseminating it as part of a smear campaign orchestrated by a rival Russian mining firm.
The allegations were part of civil suits that International Mineral Resources filed in Washington and New York accusing the Russian firm, EuroChem VolgaKaliy, of corporate espionage.
Mr. Akhmetshin said the accusations against him were false, and last year the Netherlands company withdrew all its allegations of illegal hacking. Lawyers for the Netherlands firm did not respond to phone calls or emails Friday asking for comment on the outcome of the suit.
Before it withdrew its complaints, the firm accused Mr. Akhmetshin of an elaborate scheme to steal emails and other confidential information from its computer system and disseminate it to help its Russian competitor, which it said was controlled by Andrey Melnichenko, a Russian billionaire. The firm claimed that the Russian company’s law firm paid Mr. Akhmetshin $145,000 in 2012 and 2013 to steal about 50 gigabytes of confidential information from its computer system, including the emails, personal contact lists and passport information of its executives and secret financial information about the firm.
In one email, according to the New York complaint, Mr. Akhmetshin informed the law firm that he had made good progress and the project was “up and running and it is churning up information.” Mr. Akhmetshin vehemently denied he had hacked the firm’s computers. “I have never done so, nor do I have the skills to do so,” he said in a declaration in the federal suit.