Russian President Vladimir Putin mentioned Bill Browder's name at his joint press conference with US President Donald Trump in Helsinki on Monday.

Browder is a financier behind the Magnitsky Act, a 2012 US law designed to sanction Russians suspected of carrying out human-rights offenses and freeze their assets. Several other countries also have versions of the legislation.

The law is believed to threaten much of Putin's personal wealth, which is tied up in foreign assets.

Putin offered to allow US officials to interview Russians suspected of interfering with the 2016 election in exchange for interviewing people close to Browder.

Vladimir Putin spontaneously singled out one man who poses a unique threat to the Russian leader's massive fortune at Monday's press conference, and it may hint at one of Putin's greatest weaknesses.

The man in question is Bill Browder, a US-born British financier behind the Magnitsky Act, a 2012 US law designed to sanction Russians suspected of carrying out human-rights offenses, freeze their foreign assets, and bar them from entering the US. Several countries have also passed versions of the legislation.

Many of these Russians are tied to Putin's government, and much of Putin's personal wealth is believed to be tied up in those foreign assets.

On Monday, Putin suggested allowing US officials to interview Russians suspected of interfering with the 2016 election in exchange for interviewing people close to Browder.

Russian President Vladimir Putin hands U.S. President Donald Trump (L) a World Cup football during a joint press conference after their summit on July 16, 2018 in Helsinki, Finland. Chris McGrath/Getty Images

While standing alongside US President Donald Trump, the Russian president told reporters:

"We can meet you halfway. We can make another step. We can actually permit official representatives of the United States, including the members of this very commission headed by Mr. Mueller, we can let them into the country, and they will be present at this questioning. "But in this case, there's another condition. This kind of effort should be a mutual one. Then we would expect that the Americans would reciprocate. They would question officials, including the officers of law enforcement and intelligence service of the United States whom we believe have something to do with illegal actions on the territory of Russia. And we have to request the presence of our law enforcement. "For instance, we can bring up Mr. Browder in this particular case. Business associates of Mr. Browder have earned over $1.5 billion in Russia. They never paid any taxes, neither in Russia nor in the United States, and yet the money escaped the country. They were transferred to the United States. They sent a huge amount of money, $400 million as a contribution to the campaign of Hillary Clinton. "Well, that's the personal case. It might have been legal, the contribution itself. But the way the money was earned was illegal. So we have a solid reason to believe that some intelligence officers accompanied and guided these transactions. So we have an interest of questioning them. "That could be a first step. We can also extend it. Options abound. They all can be found in an appropriate legal framework."

Browder. Getty Images/ AFP/ Bertrand Guay

Putin is 'seriously rattled'

Browder said on Monday that Putin's offhand remark showed he was "seriously rattled" by the Magnitsky Act's growing popularity around the world.

Browder wrote in Time: "Putin almost never utters the names of his enemies — except for mine, which he lately seems to utter at every opportunity. To my mind, this can only mean that he is seriously rattled."

President Barack Obama signed the Magnitsky Act into law in 2012, and other countries, including the UK, Canada, Estonia, Latvia, and Lithuania, have similar versions.

Britain amended its law in May, following the poisoning of the former Russian spy Sergei Skripal in southern England, to allow the government to impose sanctions on human-rights violators; a 2017 version allowed the government only to freeze their assets.

Browder told The Washington Post on Monday: "This is an incredibly powerful tribute to the power of the Magnitsky Act. This shows I've found Putin's Achilles' heel, that he's very rattled by it."

He added: "For a guy who's supposed to be a former KGB spy, he's got a terrible poker face."

The act was named after Browder's lawyer, Sergei Magnitsky, who Browder says was put in pretrial detention, tortured, and killed in Russian custody in 2009 after uncovering a tax-fraud scheme implicating high-level Kremlin officials and allies of Putin.

Secretary of State Hillary Clinton and Putin in March 2010. REUTERS/RIA Novosti/Pool/Alexei Nikolsky

Putin's 'Pants on Fire' Clinton allegation

Putin's claim that Browder donated $400 million to Clinton's campaign was baseless, PolitiFact and The New York Times reported.

As a British citizen, Browder cannot legally donate to US candidates in a personal capacity.

However, Ziff Brothers Investments, a New York-based investment firm Browder has worked with, previously donated to Democratic Party institutions, including Clinton's campaign, the Democratic National Committee, and the Clinton Foundation — but those sums totaled about $1.1 million, PolitiFact said.

"The exaggeration is so great, we rate this claim Pants on Fire," the fact-checking site said.

Browder, who was at one point Russia's largest foreign investor, was deported from Russia in 2005 after exposing corruption in the country. Russian authorities called him a threat to national security, and a Russian court last year sentenced Browder in absentia to nine years in prison on charges of fraud and tax evasion.

Moscow has issued at least six requests to Interpol to arrest and extradite him, Browder previously said, but the agency has routinely ignored them in the past. Browder was briefly detained in Spain in May on what he said was a Russian Interpol arrest warrant.