Washington (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.

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