In that instance, the reporter’s email had become public as part of a lawsuit. But the episode suggests Mr. Akhmetshin’s professional focus in the decades since he immigrated to the United States — and the experience that he brought to a meeting last June in New York with President Trump’s oldest son, Donald Trump Jr., his son-in-law, Jared Kushner, and the then-head of the Trump presidential campaign, Paul J. Manafort.

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The meeting was arranged on the claim that a Russian lawyer, Natalia Veselnitskaya, would provide dirt on Hillary Clinton, though in the end, Mr. Trump and his son have insisted, neither Mr. Akhmetshin nor Ms. Veselnitskaya provided anything of value.

But Mr. Akhmetshin, a gregarious, fast-talking man with a sharp sense of humor, was a skilled practitioner in the muscular Russian version of what in American politics is known as opposition research. From his base in Washington, Mr. Akhmetshin has been hired by an ever-changing roster clients, often Russians, to burnish their image and blacken that of their rivals. Some clients were close to the Kremlin. Others were its bitter foes.

Mr. Akhmetshin often warned his friends and contacts: “Nothing is secure.” It was a conviction that emerged from the chaotic and often violent corporate battles that convulsed Russia in the 1990s, when “chyorny P.R.” or “black public relations” based on stolen or fabricated documents became a powerful weapon for businessmen seeking to damage their rivals without resorting to physical threats, another frequently used tool.

The practice was rooted in the Soviet techniques of “kompromat,” the collection of compromising information by the K.G.B. against foes of the Communist Party, but reached its full flowering after the 1991 collapse of Communism and the privatization of the dark arts formerly dominated by the K.G.B.

Instead of simply examining old media reports, court records and other public documents to try to dig up dirt or embarrassing gossip, Russian-style “chyorny P.R.” has often focused on pilfering private information through hacking and physical intrusion into offices and filing cabinets.

In his own investigations over the years, Mr. Akhmetshin has acquired a reputation for obtaining email records, information from spyware and other data that appeared to be drawn from Russian hackers.

He has denied that, insisting that he conducted research for his clients and relayed accurate information to the press. A 2011 case illustrates his style.

Andrey Vavilov, a former Russian deputy finance minister and owner of an oil company, hired him to discredit a longtime adversary, a former Russian lawmaker named Ashot Egiazaryan. Mr. Vavilov had learned that Mr. Egiazaryan was seeking political asylum in the United States, and was “disgusted by this,” Mr. Akhmetshin later said in a court deposition.

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At the time, Mr. Akhmetshin was described as the director of a pro-democracy think tank called the International Eurasian Institute, although corporation records show it had been dissolved two years earlier for failure to pay taxes. He was also lobbying against the Magnitsky Act, the law which bars Russian officials suspected of human rights abuses from entering the United States.

In a defamation lawsuit later brought by Mr. Egiazaryan in a New York federal court, Mr. Akhmetshin testified that Mr. Vavilov invited him to his home in Moscow to discuss how to derail his enemy’s asylum application. “He had some cash around the house,” Mr. Akhmetshin testified. “I remember there was money in like $100 bills bags.”

He said Mr. Vavilov, who had been dogged for years in Russia by accusations of corruption, handed him a total of $70,000 to $80,000 in cash. That was the start of a concerted campaign to portray Mr. Egiazaryan as an anti-Semite in the news media and to Jewish organizations that then opposed his asylum application. “I met with people in Russia. I met with people in Washington D.C.,” Mr. Akhmetshin testified. Mr. Egiazaryan remains in the United States, but his lawyer would not comment on his asylum status.

There is no evidence that Mr. Akhmetshin’s efforts on behalf of any of his clients, whether they had close or hostile relations with the Kremlin, were illegal. Nor is there evidence that he personally engaged in the technical aspects of hacking himself. But a 2015 lawsuit filed in a New York State court put forward detailed allegations.

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An international mining company controlled by a trio of flamboyant millionaires from Kazakhstan filed a lawsuit in a New York State court charging that Mr. Akhmetshin orchestrated a hacking campaign against their company to benefit a rival Russian tycoon, Andrey Melnichenko. Each side of the dispute is known for aggressive business practices and lavish excesses.

Court papers filed in November 2015 on behalf of the Kazakhs’ company, International Mineral Resources, or I.M.R., described Mr. Akhmetshin as “a former Soviet military counterintelligence officer who moved to Washington, D.C. to become a lobbyist” and “developed a special expertise in running negative public relations campaigns.”

Mr. Akhmetshin has called this characterization misleading. He was, he said, drafted as a young man to serve in a military unit performing counterintelligence functions during the Soviet-Afghan war. As an enlisted man, he said, he never received any training in “negative public relations.”

The suit alleged that Mr. Akhmetshin was retained by EuroChem VolgaKaliy, a Russian potassium mining company controlled by Mr. Melnichenko, which was involved in a $1 billion litigation with I.M.R. The suit also names as a defendant Salisbury & Ryan, a New York law firm representing EuroChem.

The lawsuit alleged that Mr. Akhmetshin was hired to hack into the computer systems of I.M.R. and had stolen about 28,000 files, or about 50 gigabytes of data. It said the sensitive material was then distributed to journalists and others in an effort to harm the company’s reputation.

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The suit claims that Mr. Akhmetshin stole passport information, emails and personal contact lists for executives within I.M.R., along with bank account information; loan agreements; business strategy documents; board meeting minutes; drafts of market-sensitive documents; and financial forecasts and projections for the company.

An investigator for I.M.R. reported personally witnessing and overhearing a conversation at a London coffee shop during which Mr. Akhmetshin handed someone an external hard drive that Mr. Akhmetshin described as containing files obtained from I.M.R. computers, according to the lawsuit.

The lawsuit was withdrawn in 2016, and Mr. Melnichenko denied all the allegations in it.

Mr. Akhmetshin has denied that he hacked the company’s computers.

An early client of Mr. Akhmetshin’s was an opposition leader from Kazakhstan who, from self-imposed exile in the West, was waging a lonely campaign against one of Mr. Putin’s closest allies, the president of Kazakhstan, Nursultan Nazarbayev.

And in neighboring Kyrgyzstan, he helped to unravel a brazen corruption scheme by a former pro-Kremlin leader and his family that had been cheating the population, poor already, out of millions of dollars.

On other occasions, however, Mr. Akhmetshin worked on causes that clearly benefited the Kremlin, like a long-running campaign by Ms. Veselnitskaya to get Congress to repeal legislation imposing sanctions on Russian officials suspected of corruption and involvement in the death of Sergei L. Magnitsky, a Russian accountant who died in a Moscow jail in 2009.

Mr. Akhmetshin had also at times been in touch with Russian government officials, court records show.

In 2010, he submitted an op-ed column to The Washington Times on behalf of Viktor Ivanov, then the director of Russia’s anti-narcotics police, according to one of Mr. Akhmetshin’s own emails, subpoenaed as evidence in a case in the Southern District of New York. Mr. Ivanov is a longtime veteran of Russia’s security services.

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Mr. Akhmetshin has spoken openly about his service in the Soviet military in Afghanistan, and his current work throughout Russia and Central Asia.

Once, a reporter arrived for a meeting at a coffee shop in Moscow with Mr. Akhmetshin and peered around the room, trying to see where he was sitting.

Mr. Akhmetshin had come in with what amounted to a disguise: He sat smiling at a table, wearing a hat resembling a skull cap and his beard grown to an impressive length. He was headed next, he said, on an assignment to Afghanistan and had assumed the appearance of an Afghan elder.