The company behind an infamous anti-Trump dossier also worked "on behalf of the Russian government" to fight U.S. sanctions that had enraged Moscow's elite, a key witness testified to a Senate committee on Thursday.

The testimony of Bill Browder, the CEO and co-founder of Hermitage Capital, shed new light on the dealings of the controversial company Fusion GPS -- which was tied to the dossier of spurious claims about President Trump. According to Browder, the company also conducted a "smear campaign" against him in a bid to fight Russia sanctions.

Further adding to the complexity, Browder also testified that this campaign was orchestrated by Natalia Veselnitskaya – the same Russian attorney who sought the highly scrutinized Trump Tower meeting last June with Donald Trump Jr., Jared Kushner and then-Trump campaign chairman Paul Manafort.

"They were talking about a repeal of sanctions so that Russian torturers and murderers could keep their money in America," he testified.

The dramatic testimony had been blocked a day earlier by Senate Democrats, as part of delay tactics executed in protest of ObamaCare repeal efforts.

But Browder's comments could raise problems for both pro- and anti-Trump camps.

On one hand, his written testimony detailed how the Russian lawyer with whom Trump's eldest son met was involved with the same company linked to the shady dossier smearing the now-president. Trump Jr. already has faced intense bipartisan criticism for taking that meeting, apparently after being told the lawyer had dirt on Hillary Clinton

But more broadly, Browder's testimony skewered the Fusion firm itself, along with co-founder Glenn Simpson.

He said the Russian lawyer, working through an intermediary, hired Simpson to "conduct a smear campaign against me and Sergei Magnitsky in advance of a congressional hearing on the Global Magnitsky Act."

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Browder had hired the late Sergei Magnitsky to uncover details of massive financial fraud in Russia involving corrupt Russian government officials. Magnitsky was imprisoned and ultimately beaten to death by Russian officers in Moscow. In the wake of his death, the U.S. passed a law in his name that brought sanctions against Russian oligarchs suspected of money laundering.

Browder described the tactics Simpson allegedly used to discredit him and Magnitsky.

“Glenn Simpson was pitching a story to journalists that Magnitsky had not been murdered, but died of natural causes; that he wasn’t a whistleblower, but a criminal, and that the story I told Congress was incorrect,” Browder said Thursday, adding they were “unsuccessful for the most part” in getting that story written.

Left unclear in Thursday's hearing was what connection there might have been between the Russians and the anti-Trump dossier commissioned by Fusion GPS.

Browder was questioned by multiple senators Thursday on that issue -- but said he was a “bystander” in regard to the dossier and the crude allegations against Trump, as he had only read reports in the press.

“What I’m familiar with is Fusion GPS and Glenn Simpson’s role working on behalf of the Russian government to overturn the Magnitsky Act,” Browder said. “The steps they took there compromised their integrity.”

Judiciary Committee Chairman Chuck Grassley, R-Iowa, and Ranking Member Dianne Feinstein, D-Calif., originally had wanted to hear testimony this week from Simpson as well as Trump Jr. and Manafort. All three witnesses have since reached arrangements to interview with the committee, for now, in private.

A human rights advocate with the Human Rights Foundation, Thor Halvorssen, also submitted documents for the record with regard to Fusion GPS, though he did not appear at the hearing. He told Fox News the company is notorious for running smear campaigns in return for “large enough retainers.”

“It’s the sort of thing they specialize in—mainstreaming false smears, allegations that are so salacious and over the top that people assume they must be true,” Halvorssen told Fox News.

Browder’s testimony was rich with knowledge of the Russian government and corruption within, giving details of Veselnitskaya’s relationship with Russian President Vladimir Putin and other officials.

Browder said that Veselnitskaya had worked for the FSB, which is a successor to the KGB.

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“There is no such thing as a former KGB agent,” Browder said, adding that there was also “no such thing” as a former intelligence officer, mentioning Rinat Akhmetshin who attended the Trump Tower meeting with Veselnitskaya. “It’s like Hotel California—you can check out any time you like, but never leave.”

Later Thursday, Fusion GPS responded to the allegations raised in the hearing.

“Fusion GPS has complied with the law. It is a matter of public record that Fusion GPS worked for and under the supervision of an American law firm to provide support for civil litigation in New York. It was not required to register under FARA, and it did not spread false information about William Browder or Sergei Magnitsky," the company said in a statement to Fox News. “What is clear is that the president and his allies are desperately trying to smear Fusion GPS because it investigated Donald Trump's ties to Russia. They have pulled out all the stops, including this false allegation about FARA."

The statement said: "At today’s hearing, not even Mr. Browder could understand or adopt the Republicans' nonsensical argument that Russia had an agent investigate and expose the Russians’ influence on the election. Fusion GPS is cooperating with Congress and looks forward to sharing the truth.”

The June 9, 2016 meeting was billed to the Trump campaign as one to discuss Russian adoption and potentially get dirt on Clinton, but Browder said adoption was “code” for talking about the Magnitsky Act.

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“Go and ask the possible future president of the United States to repeal a major piece of human rights legislation. What were they willing to offer in return? I don’t know. And whether that offer made sense to the other side, I don’t know,” he said.

But Sen. Lindsey Graham, R-S.C., seemingly defended the Trump campaign saying, “I don’t think that in June 2016 they knew adoption meant Magnitsky Act.” Graham asked Browder about the Russian tactics and their ability to “play both sides against the middle.”

“I am not certain that they wouldn’t have made the same attempt with the Democratic candidate if there was any opening, or receptivity there—they are nonpartisan,” Browder said. “The Russians would try to bribe and blackmail anybody.”

Fox News' Catherine Herridge and Pamela K. Browne contributed to this report.