The private research firm behind a dossier full of incendiary allegations about President Trump Donald John TrumpBubba Wallace to be driver of Michael Jordan, Denny Hamlin NASCAR team Graham: GOP will confirm Trump's Supreme Court nominee before the election Southwest Airlines, unions call for six-month extension of government aid MORE is in the public crosshairs after the firm’s founder spent 10 hours behind closed doors with Senate investigators this week.

The firm, Fusion GPS, has become a political football in the increasingly complex investigations into the Trump campaign’s alleged ties to Russia.

Trump, backed by the Republican National Committee, has sought to discredit the firm by tying it to both Democrats and the Russian government and suggesting that the unsubstantiated material in the dossier helped spark the federal investigation into his campaign.

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But Sen. Richard Blumenthal (D-Conn.) is calling for founder Glenn Simpson to testify publicly before the Senate Judiciary Committee, arguing that he has “potential knowledge” about “matters relating to Trump collusion with the Russian meddling [and] possible obstruction of justice.”

Committee Republicans, meanwhile, are raising questions about any lobbying work that Fusion GPS may have done on behalf of Russian interests — allegations that have linked the company to the 2016 meeting between a Russian government lawyer and members of Trump's campaign, including Donald Trump Jr.

Here are five things to know about Fusion GPS as lawmakers and special counsel Robert Mueller continue their investigations into Russian meddling in the presidential election.

What is Fusion GPS?

Simpson, a former Wall Street Journal investigative reporter, cofounded the strategic intelligence firm with two other Journal alumni in 2012.

The company has done research for Democrats on 2012 presidential candidate Mitt Romney and for Planned Parenthood on a series of undercover videos released by anti-abortion activists, among other clients.

It has also done work for an American law firm defending Prevezon Holdings, a company owned by the son of a senior Russian government official. The U.S. government sued Prevezon in 2014, alleging fraud.

What does Fusion GPS have to do with the dossier and who were their customers?

In 2016, Fusion GPS retained a well-respected former British spy, Christopher Steele, to research any connections between then-candidate Trump and the Russian government. The firm had been hired first by Republicans during Trump’s primary run and later by Democrats to produce opposition research on Trump.

Steele produced an unconfirmed, 35-page dossier full of salacious allegations about Trump and Russia, all of which Trump has vehemently denied. The document — which circulated around Capitol Hill for months before BuzzFeed made the controversial decision to publish it in its entirety in January — has been a focal point for the intrigue surrounding the president’s relationship to Moscow.

The dossier has yet to be independently verified or publicly confirmed by U.S. intelligence officials. Simpson on Tuesday declined to tell Senate investigators who paid for the report.

Simpson’s attorney, Josh Levy, said Tuesday after the interview that the firm remains “proud” of the dossier and “stands by it.”

How important is the dossier to the investigations into Russian interference?

In late 2016, Sen. John McCain John Sidney McCainBiden's six best bets in 2016 Trump states Replacing Justice Ginsburg could depend on Arizona's next senator The Hill's Morning Report - Sponsored by Facebook - Washington on edge amid SCOTUS vacancy MORE (R-Ariz.) passed the dossier to FBI investigators, who were by then probing possible coordination between the Trump campaign and Moscow.

But it’s unclear how central the information it contained became to the bureau’s investigation, which is now being handled by the special counsel. Steele has briefed federal investigators on the sources behind his report, according to ABC News.

A summary of the memos was attached to a classified report prepared by the Obama administration assessing that the Russian government attempted to swing the 2016 election. Still, former CIA Director John Brennan has testified publicly that the summary “was not in any way used as a basis for the intelligence community assessment that was done.”

The White House has sought to cast doubt on the veracity and motivation behind the dossier’s creation and suggested that it’s the basis of the federal investigation.

“The Democrat-linked firm Fusion GPS actually took money from the Russian government while it created the phony dossier that’s been the basis for all of the Russia scandal fake news,” press secretary Sarah Huckabee Sanders said earlier this month.

Senate Judiciary Committee Chairman Chuck Grassley (R-Iowa) said during a July that the creation and circulation of the dossier “started this whole controversy.”

Fusion has argued that allies of the president are trying to smear the company because it investigated the campaign’s links to Russia.

How do the dossier and Fusion GPS relate to the Trump Jr. meeting?

During the campaign, Trump Jr. took a meeting with a Russian government lawyer who, an intermediary claimed, had dirt on then-Democratic nominee Hillary Clinton Hillary Diane Rodham ClintonJoe Biden looks to expand election battleground into Trump country Biden leads Trump by 12 points among Catholic voters: poll The Hill's Campaign Report: Biden goes on offense MORE. But participants claim that the meeting centered on the Magnitsky Act, a slate of sanctions imposed on Russia after a Russian tax accountant who had investigated the alleged Prevezon Holdings fraud was arrested and died in prison.

The lawyer, Natalia Veselnitskaya, was then lobbying for the removal of the sanctions while also working on the Prevezon case for the same law firm that had engaged Fusion’s services, according to a declaration she filed in federal court in 2016.

Did Fusion GPS violate federal lobbying rules?

Information uncovered by Fusion as part of its contract with the U.S. law firm defending Prevezon ended up in the anti-Magnitsky lobbying effort — sparking complaints that Fusion had failed to register as a lobbyist for a foreign interest under the Foreign Agents Registration Act (FARA).

Fusion said in a statement that it “worked for and under the supervision of an American law firm to provide support for civil litigation in New York” — in other words, that its work product was for the litigation related to Prevezon, not for the anti-Magnitsky lobbying campaign.

Prevezon settled the case in May for $6 million with no admission of wrongdoing. The law firm had already ended its representation of the company in late 2016, at a judge’s behest, because of a conflict of interest.

But Grassley’s committee is now investigating whether Fusion GPS failed to appropriately register as a lobbyist under FARA — still required even if a firm is paid through an intermediary. He said Thursday that the committee would hold a vote on whether to make public the transcripts from Simpson’s Tuesday interview.

Critics of the firm, including the president, have suggested that the timing of the Prevezon case is suspicious when juxtaposed against the creation of the dossier.

“It seems that Russia spent a lot of money on that false report, and that was Russian money, and I think it was Democrat money, too. You could say that was collusion,” Trump said earlier this month.