The target of a Fusion GPS opposition research investigation is accusing the firm, best known for its work on the Trump dossier, of assisting the Kremlin in its effort to have him “imprisoned and killed.”

The strong allegation comes from Bill Browder, a London-based banker who spearheaded the Magnitsky Act, a sanctions law vehemently opposed by the Russian government.

Browder made the allegation in response to the recent revelation that Fusion GPS and its founder Glenn Simpson compiled what he says is false information about him that was given to Yuri Chaika, Russia’s prosecutor general, and to Donald Trump Jr. during the infamous June 2016 Trump Tower meeting.

“If it is true that Glenn Simpson was supplying information about me to the Russian government, it’s far more serious than smear campaigning or Foreign Agents Registration Act violations,” said Browder, the head of Hermitage Capital. (RELATED: EXCLUSIVE: Oppo Researcher Behind The Trump Dossier Worked On Pro-Kremlin Lobbying Effort)

“It would mean that Simpson was assisting the Putin regime in their plot to get me back to Russia to have me imprisoned and killed.”

It was previously known that the Washington, D.C-based Fusion GPS’s investigation of Browder was beneficial to the Russian government and its effort to reverse the Magnitsky Act. But it was revealed only last week that information Fusion collected on Browder was shared directly with Chaika, whose position is the Russian equivalent of the U.S. attorney general.

Natalia Veselnitskaya, the Russian attorney who attended the Trump Tower meeting, is who provided both Chaika and Trump Jr. with the Fusion-generated information on Browder. She took a four-page report into the June 9, 2016 Trump Tower meeting, which was attended by Jared Kushner and then-campaign chairman Paul Manafort.

Veselnitskaya worked with Simpson, a former Wall Street Journal reporter, on the anti-Browder project. Reuters reported on Thursday that Simpson compiled the research for the Trump Tower memo. And NBC News reported on Friday that similar research put together by Simpson was given by Veselnitskaya to Chaika in Oct. 2015. (RELATED: Russian Lawyer Took Fusion GPS Report To Trump Tower, Russia’s General Prosecutor)

Browder, who was born in Chicago but is a British citizen, is a target of the Kremlin’s ire because of his role in pushing through the Magnitsky Act, which passed Congress in 2012. The Russian government recently issued an Interpol arrest in an attempt to apprehend Browder. Vladimir Putin reportedly approved the move against the 53-year-old money manager, who wrote a book about his travails called “Red Notice,” a reference to the term used for Interpol warrants.

Fusion GPS, which is based in Washington, D.C., became involved in the Browder matter after being hired by BakerHostetler, a law firm representing Denis Katsyv, a Russian businessman who faced penalties because of the Magnitsky Act.

Simpson worked closely on the investigation with the Moscow-based Veselnitskaya, who represents Katsyv and his firm, Prevezon Holdings.

Veselnitskaya’s affiliation with Simpson is one of the stranger story lines arising from both the Trump Tower meeting and the dossier, which was funded by the Clinton campaign and DNC.

Theories have circulated that Fusion GPS arranged the Trump Tower meeting in order to compromise the Trump campaign or that Simpson relayed details of the meeting provided by Veselnitskaya to Christopher Steele, the former British spy who wrote the dossier.

Fusion was being paid for its dossier work at the same time it was being paid by BakerHostetler to investigate Browder.

Adding even more fuel to those theories was a report from Fox News this week that Simpson and Veselnitskaya met just before and just after she attended the Trump Tower session. (RELATED: Fusion GPS Founder Met With Russian Lawyer Before And After Trump Tower Meeting)

The pair were together in a Manhattan federal court room just before the meeting for a hearing related to the Katsyv/Prevezon case.

Fusion’s lawyer has denied that Simpson knew of the Trump Tower meeting.

The dossier, published by BuzzFeed in January, has fueled allegations that the Trump campaign colluded with the Russian government during the presidential campaign.

Trump Jr. accepted the meeting after being told by an acquaintance that a “Russian government attorney” — now known to be Veselnitskaya — would provide damaging information about Hillary Clinton.

Trump Jr. accepted with enthusiasm, writing back, “I love it.”

Trump Jr. and Veselnitskaya have both said that no substantive information was provided about Clinton. Instead, both sides say that Veselnitskaya wanted to discuss the Magnitsky Act.

The memo that she took into the meeting includes only one passing reference to the Clinton campaign. It suggested that the Clinton campaign took illegal campaign contributions from Ziff Brothers, an investment fund affiliated with Browder.

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The Trump Tower memo contains a slew of allegations about Browder, all of which he denies. The document also embraces Kremlin rhetoric, accusing Browder of manipulating U.S. lawmakers who have “earned their political dividends on fomenting a cold war with Russia to use this false story.”

“This Act was in fact the beginning of a new round of the Cold War between the United States and Russia, putting on the scales the interests of a group of scammers and interstate relations,” reads the memo.

In an interview with NBC, Veselnitskaya said that she worked as a consultant to Chaika on the Browder case. The admission is significant because it ties Veselnitskaya to the upper echelons of the Kremlin, a link which she has previously denied.

“I was in effect, the primary source of this information for the Russian Prosecutor General’s office. They then published the facts I uncovered,” she told NBC News in an Aug. 31 interview that the network did not release until last week.

Fusion provided the information on Browder to Veselnitskaya in 2014, she said.

It is unclear whether Fusion GPS wrote the memos provided to Chaika and Trump Jr. or merely provided the research for the documents. It is also not known whether Fusion was aware that Veselnitskaya provided its research to Chaika and Trump Jr.

Simpson did not respond to emails, text messages or a voice mail seeking comment.

Browder has said that his former lawyer, Sergey Magnitsky, the namesake of the Magnitsky Act, was murdered in a Russian prison in 2009 because he had uncovered a $230 million money laundering scheme involving Russian businessmen.

The Russian government, including Chaika, has disputed that, and accuses Browder of fabricating the Magnitsky story in order to cover up for his own financial misdealings. Veselnitskaya appears to have taken the case personally. She frequently posts about Browder on social media.

Fusion research given to Chaika was used by the Kremlin official to call for legal assistance from the U.S. government in going after Browder.

Browder pushed back against the Fusion GPS attacks last year, filing a complaint with the Justice Department accusing Fusion GPS and Simpson of working as foreign agents of Russia but failing to properly register under the Foreign Agents Registration Act (FARA).

Lobbyists and consultants who provide services to foreign governments are required to register under FARA.

Fusion GPS has rejected the idea that it needed to register as a foreign agent.

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