Akhmetshin registered last year as a lobbyist for the Human Rights Accountability Global Initiative Foundation, which calls itself "a non-governmental organization established in Washington, D.C., to help restart American adoption of Russian children," according to its website. He was paid $10,000 in the second quarter of 2016 to lobby on "foreign adoption reinstatement" on behalf of the foundation, congressional records say.

His lobbying activity caught the attention of Sen. Chuck Grassley, R-Iowa, chairman of the Senate Judiciary Committee. In a letter this week to Trump administration officials asking how Veselnitskaya was able to enter the United States, Grassley wrote that, "in March of this year, the committee began investigating the Justice Department's response" to a formal complaint that "alleged that a group of unregistered foreign agents worked in the U.S. on behalf of Russian interests to undermine the Magnitsky Act and the Global Magnitsky Act."

That complaint was filed last year by Hermitage Capital Management CEO William Browder, who is scheduled to testify on Wednesday before Grassley and the Judiciary Committee. The panel is looking at Justice Department enforcement of a law requiring that agents acting in the interests of a foreign government disclose those relationships.

In April, Grassley sent a separate letter to Department of Homeland Security Secretary John Kelly asking for more information on Akhmetshin. Grassley said the lobbyist "has been accused of acting as an unregistered foreign agent for Russian interests and apparently has ties to Russian intelligence."

The Russians who met with Trump Jr. were hoping to discuss the Magnitsky Act, which drew bipartisan support in Congress and was signed by President Barack Obama in late 2012. The law was named for Russian accountant Sergei Magnitsky, who died in a Moscow prison in 2009 after investigating fraud involving Russian tax officials.



In 2012, at the urging of Browder, Magnitsky's former employer, Washington passed the law, which froze the U.S. assets of Russian officials said to have been involved in the accountant's detention. Moscow then barred Americans from adopting Russian children.

In a civil lawsuit filed in 2013, U.S. prosecutors alleged that Magnitsky had uncovered an elaborate money laundering scheme involving some proceeds of a $230 million Russian tax refund fraud plot. In May, prosecutors settled the case for $6 million. In the settlement agreement, they said that none of the defendants had a role in the death of Magnitsky.

Browder's complaint to the Justice Department alleges that Akhmetshin is among the lobbyists who took part in the Human Rights Accountability foundation's efforts to undermine the Magnitsky Act. Browder contended that several individuals did so in the interest of the Russian state without following the proper filing requirements of the Lobbying Disclosure Act and Foreign Agents Registration Act.

The Foreign Agents Registration Act requires that lobbyists acting in the interests of a foreign government must disclose that relationship with that government. CNBC could find no record of Akhmetshin having registered as a foreign agent.

In Akhmetshin's case, Browder's complaint partly cites a Daily Beast report about events that took place last year after the Senate's passage of the "global" Magnitsky bill, which aimed to extend the sanctions framework worldwide. The report reads:

According to a U.S. congressional staffer, former California Rep. Ron Dellums and someone named Rinat Akhmetshin showed up Tuesday without an appointment.

"They said they were lobbying on behalf of a Russian company called Prevezon and asked us to delay the Global Magnitsky Act or at least remove Magnitsky from the name," the staffer said. "Mr. Dellums said it was a shame that this bill has made it so Russian orphans cannot be adopted by Americans."

Veselnitskaya, the lawyer at the meeting with Trump Jr., has previously represented Prevezon.

Browder told CNBC on Friday that the effort to undermine the Magnitsky Act "is a highly resourced project that goes right up to the top of the Russian government for a major issue that Putin wants to have resolved."