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FILE - AUGUST 6: New York City comptroller candidate and former madam Kristin Davis was arrested August 5, 2013 in Manhattan for allegedly selling hundreds of prescription pills to a drug dealer. HEMPSTEAD, NY - OCTOBER 18: Kristen Davis of the Anti-Prohibition Party looks on at the gubernatorial debate at Hofstra University October 18, 2010 in Hempstead, New York. The two main candidates, Andrew Cuomo and Carl Paladino, appeared onstage with candidates Kristen Davis of the Anti-Prohibition Party, Warren Redlich of the Libertarian Party, Howie Hawkins of the Green Party, Jimmy McMillan of the Rent is 2 Damn High Party, and Charles Barron of the Freedom Party. (Photo by Mario Tama/Getty Images)
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(CNN) —  

Kristin Davis, also known as the ‘Manhattan Madam,’ will testify before a grand jury in special counsel Robert Mueller’s investigation on Friday, her spokeswoman Lainie Speiser told CNN.

Davis met with Mueller’s team last week for a voluntary interview, sources familiar with the situation previously told CNN. It is not clear what the focus of that interview was or how Davis may fit into the investigation into Russian interference in the 2016 election and potential coordination between Trump associates and the Russian government. But investigators have been very interested in Roger Stone, with whom Davis has a close personal relationship and has worked for in the past.

The grand jury testimony on Friday comes as Mueller’s team faces its first major test in court with the trial of former Trump campaign chairman Paul Manafort in the Eastern District of Virginia, where Manafort faces charges of bank fraud, tax evasion and other financial crimes. He has plead not guilty to all charges.

Davis once ran a high-end prostitution ring and went to jail as part of the scandal surrounding then-Democratic New York Gov. Eliot Spitzer. Davis was not involved in the specific prostitution service Spitzer used that led to his resignation in 2008.

She has known Stone, a longtime associate of President Donald Trump, for a decade. Stone acted as a campaign strategist for Davis in 2010 when she ran for New York governor on a platform that included legalizing marijuana and prostitution. In late 2016, she joined Stone’s payroll to assist in email blasts, media bookings and other clerical matters, according to a source familiar with the situation.

Stone called Davis “a longtime friend and associate,” in a statement to CNN last week, but said, “She knows nothing about Russian Collusion, WikiLeaks collaboration or any other impropriety related to the 2016 election which I thought was the subject of this probe.” Stone said that he was “highly confident she will testify truthfully if called upon to do so.”

Stone told CNN’s Anderson Cooper earlier this week that he has not been contacted by the special counsel’s office and said that while he “would not rule out cooperating,” with the investigation, he would not testify against Trump.

Trump has long insisted there was no collusion with Russia and has slammed the probe as a “witch hunt.”

The President has escalated his attacks on the investigation – publicly calling on his Attorney General Jeff Sessions to shut down the probe, though the White House has said he was merely expressing an opinion, not giving an order – as his legal team continues its prolonged back-and-forth in negotiations with the Mueller team over whether Trump will sit for an interview as part of the investigation.

CNN’s Katelyn Polantz contributed to this report.