‘Manhattan Madam’ expected to appear before Mueller grand jury

Kristin Davis, a former prostitution-ring operator, is expected to testify.

Kristin Davis, a former upscale-prostitution-ring operator once known as the "Manhattan Madam," is expected to testify before a grand jury in Washington, D.C., Friday in connection to special counsel Robert Mueller's probe into Russian meddling in the 2016 presidential election.

Three sources confirmed to ABC News that Davis was asked to interview with special counsel investigators last month, and her spokesperson said she completed an "informal" interview with members of Mueller's team last week.

Davis has close ties to Roger Stone, a longtime political campaign operative whose resume dates back to the Nixon administration. He also served as an adviser to Donald Trump's 2016 presidential campaign, but left amid controversy in 2015.

An associate of Stone’s, Andrew Miller, was supposed to testify before the grand jury but failed to appear and was held in contempt.

Miller’s attorney, Paul Kamenar, confirmed to ABC News his client is appealing a ruling that required him to comply with the special counsel’s subpoena.

For procedural reasons, Kamenar said, Miller needed to be held in contempt in order to initiate the appeal.

Sources told ABC News last month that Mueller's team was pushing for more information on whether Stone communicated with hackers who targeted DNC computers. At least nine people associated with Stone have been contacted by the special counsel.

Following Davis' previous meeting with investigators, Stone said in a statement: "Kristin Davis is a longtime friend and associate of mine. She is a brilliant businesswoman who paid her debt to society and remade her life. I am the Godfather to her 2 year old son."

Stone added, "[Davis] knows nothing about alleged Russian Collusion, WikiLeaks collaboration, or any other impropriety related to the 2016 election which I though was the subject of this probe. I understand to she appeared voluntarily. I am highly confident she will testify truthfully is called to do so."

Davis made headlines in 2008 when she was arrested as part of an investigation into former New York governor Elliot Spitzer's solicitation of prostitutes. Davis claimed Spitzer was one of her numerous high-profile clients, which he denied. Davis pleaded guilty to promoting prostitution, and spent about four months in New York's Rikers Island prison complex. Following her release, she ran unsuccessfully for governor of New York.

In 2014, Davis ran into trouble with the law again. She pleaded guilty to selling prescription drugs in 2014 and was sentenced to two years in prison. She was released in 2016, after Stone had left the Trump campaign.