When Russian lawyer Natalia Veselnitskaya met with top advisers to Donald Trump’s campaign last summer at Trump Tower, the attorney arrived prepared to hand over a plastic envelope of documents.

No one has revealed where that folder is today.

But one name reportedly flagged in the file is William Browder. The US-born hedge fund manager was once one of the biggest foreign investors in Russia. Then he fell out with Russian authorities and, in 2009, his lawyer Sergei Magnitsky died under mysterious circumstances in a Russian prison.

Browder is set to testify Wednesday before the Senate Judiciary Committee, along with Donald Trump Jr. and former Trump campaign manager Paul Manafort. Lawmakers could ask all three about the contents of Veselnitskaya's file.

"I don't think we know specifically what was in the envelope, but we know that Natalia Veselnitskaya had been on a mission on behalf of the general prosecutor of Russia, on behalf of Vladimir Putin, to try to dismantle and repeal the Magnitsky Act, which imposes sanctions on Russian torturers and murderers in the United States," Browder says. "In order to do that, one of her big tasks was to try to discredit me."

The Senate Judiciary Committee is so determined to find out what happened in the Trump Tower meeting with Veselnitskaya that, on Monday night, they sent Manafort a subpoena.

Veselnitskaya's folder and its contents may ultimately be subpoenaed as well.

"I know in vague terms that it was a folder constructed of various stale and disproven allegations against me by the Russian government for tax evasion that they had made many years ago as part of their efforts to try to discredit me," Browder says. "[Veselnitskaya] was basically sort of rehashing, reusing that and trying to hand that around to people in Trump Tower and other places."

Veselnitskaya has served as an attorney for the FSB state security service, a successor to the KGB. She represented the FSB in a legal battle over ownership of an upscale property in northwest Moscow between 2005 and 2013.

The Russian government and Veselnitskaya deny that she represented the Kremlin while meeting with the then-candidate's son, son-in-law and campaign manager. In response to the allegations, the attorney recently posted on Facebook, "You'll be surprised to find among my clients Russian businessmen ... as well as citizens and companies that had to defend themselves from accusations from the state."

Browder rejects her denial.

"What all this says to me is that she is a senior operative for Russian state interests and that she takes her instructions from a very high level," Browder says. "Any of her statements that she is just a simple lawyer having had this strange [Trump Tower] meeting on her own volition is complete nonsense."